Originally Posted by
klogaroth
HFC - after Kilrogg was a near 2,000 drop
BRF - after Gruul was a ~1,700 drop
Highmaul - after Kargath was a ~5,000 drop
SoO - after Fallen Protectors was a ~2,400 drop
ToT - after Jin'rokh was a ~4,400 drop
HoF - after Ta'yak was a ~1,500 drop
MSV - after Stone Guard was a 2,100 ~drop
DS - after Morchok was a ~10,000 drop
FL - after Shannox was a ~6,500 drop
T11 - after Halfus was a ~10,000 drop
ICC - after Gunship was a near 25,000 drop
TotGC - after Jaraxxus was a ~2,400 drop
Ulduar - after Heartbreaker there was a ~2700 drop to the next hard mode, I Choose you Molgeim, which wasn't even the hardest mode of the fight.
Notes on rounding - ~ indicates rounded down, near indicates rounded up due to being very close to the next 100
The exact point in which the first big dip in completion in a raid varies, some let you get further in before the difficulty spike occurs, but most raids have a boss early on where a lot of people get stuck. Whether that's because (very subjective) the early bosses are too easy or the later bosses are too hard also varies. It would be easy to argue that the Gunship drop, for example, was as much to do with the ease of Gunship as it was the difficulty of subsequent bosses. The result is the same though, a difficulty spike early on in the instance, where things start to get harder and kill numbers drop off.
None of these gaps can be blamed on the AP system. Given that these are end of tier numbers there is a difference to the exact situation we're currently in, but I would expect current trends to continue on as the tier progresses, giving similar results at the end of the tier.
It's not all that unusual for there to be one or more bosses at the start of mythic(or heroic back then) that aren't much of a step up, and sometimes a step down, from the last boss of heroic (or normal back then). After these, as mentioned above, the difficulty hits another level and kills drop off.