The fastest Macintosh in the world right now should be a Microsoft X360 developer unit running a three-way, 3.2Ghz, PowerG5 successor on a 22.5GB/S bus. The only problem with it, of course, is that Microsoft's nexus DRM implementation prevents it from running MacOS X.
That's fixable, although probably not through the usual software porting methods. No, what's needed to get around the nexus, for MacOS X as well as Linux, is re-manufacturing. Buy X360s in volume, then rip out the internals and use them as plug in components on a buyer assembled PC that ships with a custom Linux or Darwin/BSD CD and runs anything the buyer wants it to.
This will require a custom GCC and Glib to account for operational change from the G5 and any check instructions embedded in the software, but any set of DRM controls, including the three way nexus, can be defeated in this way.
You might reasonably expect this to be expensive, but PowerPC has consistently been cheaper than Intel. A 3-way, six thread, 3.2Ghz X360 CPU board, complete with 256MB pre-installed, can be ripped out for less than $300 - about the same as the retail price on a P4-3.4E/800 and substantially less than half the volume price on Intel's P4-3.46GHz/1066 Extreme CPU.
It's not like there isn't a market so the question is whether or not Apple, IbM, and/or Microsoft can do much to stop it. That's a question for the lawyers - but the answer has to be less than obvious given that all involved would have legally purchased, or licensed, everything involved.
Meanwhile, the irony is pretty clear: if you really, really want a next generation, superfast, Mac: get out your tool kit and think about buying the IBM hardware you need, from Microsoft.