ToME: the Tales of Maj'Eyal

since mindstar mastery become sort of necessity of mindpower users, mages should get something similar.
At the end of game, mindpower users could get around 80-90 mindpower from mindstars, but mage only can get 40-50 spellpower from staff, so buff staff mastery a little won't be overpowered.

You're assuming level playing field to begin with. Spells seem to frequently pull much higher damage then mind powers, though this may be my limited experience. Additionally, a LOT of good arcane armors boost spellpower, but I don't seem to run across a many mindpower+ armors. Again, your mileage may vary.

Yes the way mindpower and spellpower works is different and changing it just for the sake of symmetry is bad