So I read this and did a little of my own checking.
I think perhaps an explanation more detailed than "stats don't matter" is warranted.
It looked to me like all gear higher than 34 gets set to 34.
Taking off a normal clothing slot did not change stats appropriately for what was on the item, so it appears it's wholly templated.
I didn't test if weapons or trinks are an exception to this, nor if using ilvl<34 gear makes any difference.
Just what the template is seems to be mastery heavy for my priest at least.
Gems and such don't seem to have any effect.
Didn't test enchants, suspect they don't either, but would be curious to check stat vs proc, weapon vs non to be sure.
As one would expect, no hoa n artifact gear fx; don't have the epic bfa cloak upgraded enough to check it's one pvp-allowed effect.
Val'anyr's bubble effect is a no. Didn't look at too many weapons, but so far no reason to believe anything but templated.
None of this contradicts what was said earlier, but made what was going on a little clearer to me.
Spells that are usable by 29s are usable but not spells higher than.
Choosing a pvp talent that's not normally accessible does not seem to work.
The 2nd, lvl30 pvp talent also doesn't seem to take effect.
alchemy-like item
https://www.wowhead.com/item=118922/oralius-whispering-crystal doesn't work.
Personal verdict: meh. Viable but not better than a normal twink. Makes having fun with higher lev buddies possible with no work on their part. Some of the spells gained can be pretty handy; it's nice to mind-sear out a rogue and have speedbubbles as a priest anyway.