Using Spark of Zandalari and the shieldwall trinket (Relix of Xuen is in my bag)
If I want the brutal talisman of the shadopan but it puts me at like 8.6% expertise is that too high to take above cap and replace the operation shieldwall rep trinket or is the upgrade still better do you think?
100% depends. Basically, is the gain from the new gear you get worth "losing" up to a few hundred secondary stats. It's much harder to judge with trinkets too. Gear it's simple to math out. Trinkets, not as much. (At least not for me.) Honestly, I'd say simcraft it.
From logs, Spark of Zandalar is a pitiful joke of a trinket, still being worked on (I hope). You'll see more benefit dropping for Relic or Final Orders.
From logs, Spark of Zandalar runs about 80% uptime of a spark. However, the buff when 10 stack, is called Zandalari Warrior, which you'll be likely to not even get 10% uptime of the buff the entire fight. Needs to fixed or mended.
As for how much expertise, I'd Imagine being as close as you can while reforging out of expertise and still being at or near (or above) to 7.5% is bested.
Yeah it really depends. I use brutal talisman and it puts me at least around 1% over cap currently (without counting my +1% expertise from axes, praying for the sword drops) what is around 300 rating alone if i'm not wrong. However I don't really mind, because the trinket is still nice a upgrade over relic/final orders. Also most the fights can make the melee dps to a have akward position where you can get stuff parried, so having slight overcap of expertise during progression of normal tier isn't that bad of a handicap imho.
In most cases that extra expertise usually transfers into mastery/haste anyway, because you will prio crit gear. So losing for example ~100-300 rating from "overcapped" expertise doesn't show it in any dps meters for real. The proc is still good enough alone for burst, so I would at least test it...
I've no idea about Spark of Zandalari but if the above poster is right, then it sounds like a waste to use atm (until it gets fixed).