Having lower % values in heroics doesn't make sense to me
Like 5% of a 500 HP toon is +25 HP, it's basically just a different stacking False Life or something. 15% is at least a meaningful amount. Yeah it's numerically a lot of HP for a 1k level 15 toon, but it probably won't change the cases where they die vs the cases where they don't very often (going afk, plagued + cc + champs, veng stuff? Hmm). It is, however, more meaningful to a vet vs "wow who cares about +11 sacred false life" or some other stacking effect.
I'll probably get and use the cloak if I don't need stats, but at lvl 15 I have a whole set of +8/+3 mental stat cloaks so might depend a bit. It's a great choice if you have a free slot lol.