While the current balance has its flaws, I'm not sure that a straight percentage increase would be a strict improvement. *tries to do the math* wait, they're surprisingly similar? But the current one is much more potent in the end (given I've modelled it correctly; it's bedtime)
https://www.wolframalpha.com/input?i=plot+f%28x%29%3D15-x+vs+g%28x%29%3D15*0.9%5...
I did enjoy the sorceror finally getting off his butt at the higher levels, in particular the final one where the relative speedup is yuuge.