Well, think about it, the processing power of all computers have increased dramatically since the days of MS-Dos or Win95. Are your computers and applications now running faster then back then? Of course not. Everything is still as slow as it used to be because the demands of a new OS and applications have increased as well.
Similarly for current demands the Axe III will run a lot smoother, but I'm sure artists will find a way to slow it down. Just like they did with everything else. The more you can do with it, the more you start doing with it. And I reckon Edge will still need 4. One for his main sound, one for shimmer, and two for backup. Because life on the road is always hard on gear. And maybe a few II's as well, as fromw hat I hear presets on the II can't be ported over to the III. So if he has a few sounds that he tweaked to perfection on the II he may feel he has to have one (or two (backup after all)) in his rig just for that. It's The Edge after all.