Try State of the Art or Nine Fingers by Spaceballs at some point. They're just pre-AGA demos, but I've never seen them run full speed in WinUAE with perfect emulation (of course you may need an A500 to really compare).
As for drivers... I don't know where you live and what sort of shortage of sound cards you may have there, but the most normal soundcards around here are supported by OS4. Have a look-see here:
OS4 depot driver section.
Various Soundblasters, Audigy, Audigy 2, Terratec Aureon Sky, Phase28, M-Audio Revolution 7.1 and ESI Juli etc as well as the cheapo typical on-board chips are supported.
Graphics drivers will eventually be a problem I suppose, but I expect it will also be handled before it reaches critical - heck, these days you can even still find voodoo 3 cards if you look hard enough.
Regarding the responsive Windows-bit... I'll believe that when I see it. Microsoft have never been the optimizing kind of company and I don't think they'll start now. In fact I bet manufacturers would love them to drive hardware requirements through the roof, so they can sell stuff again to the average consumer And regarding WinFX... well, it's already in Tiger (similar technology anyway) and the difference isn't as great as one would expect. AmigaOS 4 is definitely still faster - by far (on slower hardware).
The application test you mention could be interesting, but it would have to be very all-round. The reasoning behind this: Let's say we script a fast Windows machine and an old Amiga (68060-based for fairness sake) to perform the same actions in an application. I bet the PC would win. However, I also bet that the Amiga would be more comfortable to use while working on this, meaning you could easily do other stuff in the meanwhile. Now don't get me wrong. I know Windows can do things simultaneously, but it does tend to fall to its knees as soon as it's computationally heavy and lose response times (that aren't that fast in the first place).
My Powerbook is much better at this scenario than my PC (and the Powerbook is a lowly 867MHz G4), but the AmigaOne is much much much faster than the Powerbook (compared to the gap between the PC and the Powerbook).
I'm never driven to the brink of madness by having to wait for the interface to respond on the Amiga. I am on the other platforms - particularly Windows. I've got Ubuntu installed and that performs a bit better overall, even though it has some issues of its own.
As for which of the three systems are the fastest... Sure that would be interesting, but not something I would use to decide by itself. I think having the choice between them regardless is more important. Some people have an x86 rig that they want to use or want to get involved with hacking their own version of AmigaOS. AROS is for them. Some want the straight evolution of AmigaOS and OS4 is for them. Some prefer the alternative approach with boxing and somewhat higher legacy compatibility (even though both systems are doing well, MorphOS goes further to ensure 3.x compatibility while OS4 is trying to evolve while keeping 3.x compatibility too - which means some things will break though - in my experience most software works though).
As long as it's fairly easy to support all the platforms from a developer point of view, the variations between the platforms are less important - except for the competition aspect where each try to beat the others; a good thing in my book.
