hmm i don't know if that's a good suggestion. as it is now, getting water 3 for the mantle buff is really nice, since it also makes spellcasting for non-mage heroes more viable. i like the synergy of that. get the champ with the water spellbooks to water 3 so now your defender with earth spell book can use shockwave more casually and your life magic / commander can use his healing spells more without draining all your mana.
by moving the percentages to the mage tree, you basically weaken hybrid characters that use some martial power or organizational skills combined with magics from their innate spellbook(s). IMHO those champions are more interesting than a min/max hero who only picks the required stuff to get to the OP perks at the end of the tree ASAP.
and of course, it's also a nerf to mages that pick the summoning route, since they probably wont' grab half a dozen traits from the nuke spell line (spell mastery etc.) just to get the mana discount.