Going by the average of 3.5, four dies would get us: 3*3*4*4 = 144, very much overshooting even effective level 9 spells.

So I guess you need 5-8 dies to hit that sweet spot of effective level 9 spells. Imagine at level 7, being able to apply 6 metamagics to a spell without any drawbacks? For 3 feats?

And then there is Calculating Mind, to improve the odds? I dunno but its insane to think there are feats to boosts an already broken feat.