Well part of the purpose of this thread is for knowledge, and everyone's input is welcome. There are many details that can easily be missed that carry weight.
Like I forgot that dragons also inputed into the Kirin'tor - Krasus for example - how prolific he was is unknown. He wasn't a blue, and dragons don't know everything he is significant. On the surface, the Kirin'tor should pale in comparison to the nightborne/highborne group - it is a young body, sure it is a magical society ,b ut it's piecing together a lot of things and fragments that the night elven group have the full extent off in many areas - so you would think both elven bodies, night and blood elf should blow them away.
But it doesn't seem to work like that. Kael'thas studies with the Kirin'tor and is a member of the 6 at one point - to me this makes me wonder how relatively powerful the kirin'tor is to the high elf/blood elf group - I don't know, but that high elves were active members and teachers there means a lot of their knowledge is probably shared there. Does that put the Kirin'tor above the high/blood elves?:
Add to that blood elf knowledge gained via Illidan and the legion and the naaru tech before they got redeemed, and they have advantages not even the nightborne/highborne have.
Power vs knowledge? Power seems to matter a lot even if you don't have much knowledge - hence Orc warlocks in the first/2nd war being a match for mages or shaman going toe to toe with far more ancient/knowledgeable studied groups like druids or mages etc. We beat Archimonde in WoD - granted we've grown alot too, to mythical heights since WC3 - still. who knows
Some TLDR - ramblings aloud.
The council of Tirisfal and the Guardian block are now folded into the Kirin'tor - Medivh is gone, the Violet Eye, part of the Kirin'tor holds its key and Khadgar is now the master of the place he has locked up and now leads the Kirin'tor - i feel a huge story opportunity was missed here, because in the Last Guardian, the rivalry between the two groups was quite apparent, and now they're one happy family. This adds all the mysteries of the Order of Tirisfal and the Guardian to the Kirin'tor.
Thought the Shen'drelar may have fallen, the rest of the highborne in the nightborne and the Moonguard still remain and are very much strong and up there. They've had their own advancement in 10k years, the nightborne and the rest of the highborne based magical groups on Broken Isles should carry a phenomenal amount of magical knowledge that hasn't fallen to ruin, and whiles most of the shen'drelar might be out of their mind consumed with more magic and the effects Immol'thar has had on them, the uncorrupted ones are not. How do they weigh in? Shen'drelar advancement would have halted a few thousand years ago when the place started falling into ruin due to immol'thar - we see them inadequate against the blood elves - but that may have been only from a combat point of view - not all magical knowledge as there would be many areas and mysteries of magic completely lost and not yet recovered by neither high elves/nor kirin'tor - and the blues who might know, have not really played a part in their affairs with the exception of Kalecgos. WE however find no such lapse with the highborne led groups of the Broken Isles i.e. the nightborne/moonguard and possibly the cursed Court of Farondis - they pressed on with magical knowledge abuse/corruption free.
However as we can see quite visibly their magical acumen is most impressive, they have not been tested like the Kirin'tor/Blood elves have been. This may not mean much at all though, or not as significant as it was with the shen'drelar but there is that. IT could mean they are more advanced having not had to deal with wars to slow them down, but not quite as advanced in the magical warfare department as they may have been. Afterall, given that we beat Illidan/Lich King/Archimonde/Deathwing beings of immense power and knowledge, being old and incredibly knowledge does not mean we can't be effective against you. And we do raid the nighthold afterall and win, despite their superior magical knowledge - we aided by the nightborne and moonguard to be fair - but we do the encounter.
there is another thing also, The blood elves did gain a lot of magical knowledge during the reckless era. When they learnt to utilise very advanced Naaru technology, at a level that even surprised the draenei - though they had Illidan and Legion help - still it was something and as a group would have gained a lot - remember Kael'thas' greatest and brightest were the ones that abandoned him and turned to A'dal forming the Scryers and turning the blood elves around , all those who were going to Outland to join Kael'thas instead turn to find him and his betrayal is relayed to Silvermoon that eventually stands against him.
If Illidan though could be so powerful, imagine what other 10k year old highborne/nightborne like Thalyssra, Elisande, Moonguard people would be since they continued advancing while he sat in a cell, and he operates at a level beyond any Kirin'tor mage .. but part of the problem here is
we do not know how magical knowledge and power are related. The only magical acumen that bears weight for warcraft is your ability to fight, and it seems that people with much lesser knowledge can beat people with much greater ones - or it's not so much knowledge based as it is power and skill based. Knowledge matters when it comes to other things like building an advanced society, having the magic equivalent of irl technology at a level higher than anyone else, this is where the vast weight of knowledge is brought to bear, but when it comes to fighting, it's advantage is much smaller even though it has an advantage, sort of like the new pvp normalization where your level/ability/gear don't give you as much an advantage in it as it does outside. This is the only thing that can explain how despite how much younger we are we are able to do things against beings who should be able to obliterate.
Furthermore, we don't know how knowledge is transfered, the night elves used ways of transferring vast amounts of knowledge to speedily train people, the arcane boosts your intellect significantly too, so you are able to be a lot smarter which I can only assume means much more memory retention, speed of thought and calculation, able to process information and quickly solve things. We find out while using arcane constructs holds a significant advantage, the high elves stopped using them over a thousand years ago because they also posed a security issue that could be exploited. Also demonstrating military ability vs knowledge aren't always linearly related. Whiles it does appear most magical tomes need to be read, like books, it also appears most have more to them too or faster ways of acquiring knowledge, this could actually allow much younger groups to benefit or gain a lot of knowledge faster as long as the ancient groups who had the knowledge share.
i.e. everyone can get up to date and the differences may just lie in the size and diversity of the magical communities. races like the Blood elves and the High/nightborne have entire communities based around magic, but that is waht the kirin'tor is too, it's not part of a nation that does other things, it's a magically based nation.
Finally strength of power seems to account for the lionshare of capability, how much of punch your fireball packs is significant, you may tons of knowledge more but if his fireball packs much more of punch than yours - he'll kill you. Your knowledge might help find ways of breaking down the spell or protecting yourself he doesn't know but power is the pedigree. Orc warlocks hardly knew the complexities of magic compared to their elven and human magi counterparts, but they were able to sufficiently challenge them despite their superior knowledge because of the raw power of the magic they wielded. This is also significant.