I don't have any doubt that sharing a larger transformer can provide benefit to power-hungry main or center speakers. On the other hand, if your budget is getting tight and you don't plan to add two more speakers for a long while, if you buy five channels now (with a 7500) and add a pair of M2200's to drive surround back speakers, you'll be using 200-watt monoblocks to drive surrounds that will be asked to do far less than the mains and center. In that scenario, even very inefficient surrounds would never call for more power than a 200-watt monoblock could provide. If postponing the purchase of those last two channels lets you buy nicer speakers or a better surround processor or some new DVD's (or groceries) now, I think you'd be better served with that now and in the long term.