Depends on which XP cap remover you speak about. There are 2 of them - one is up to lvl 40 and another up to lvl 50. With ToB, there's no need for the lvl 40 one as the ToB supposedly will let you do this anyway. And besides, getting to lvl 40 already and then trying to get to lvl 50 will not yield anything that big. Don't think that you will get any more special abilities (additional and new ones) past the XP cap which is in ToB, probably just miniscule additional amount of HP and occasional Lore point. So no point really. Lvl 40 with the ToB is really all the power you will ever need
I personally used the "lvl 40" XP cap remover with SoA and at the end, my human berserker was lvl 28 or 29. But starting from the XP cap of SoA, I received only +3 HP and +1 Lore with each lvl up, nothing more, so nothing too big really..