Aha, possible houserule: if you spend Edge to add to a die pool, then you add your Edge attribute to the cap for successes.
IOW, if you have Magic 5, Sorcery 6, and Power Focus 2 ... your DP is 13. Casting a F3 spell, you're normally capped at 3 successes.
But if you use a point of Edge, to add your Edge attribute of 4 to teh DP? You cap also goes up by 4 (from 3 to 7).
It's nearly as limited as the "different color die" method, but retains the vast majority of the "just roll and don't worry about success caps" approach. Best of both worlds, I think.
Anyone see any serious problems with that?