Here's my two cents here
It seems like modern USB2 or firewire would be more than fast enough for anything that ever took place in a real Amiga's memory bus-think around 30 megs/sec which is well within firewire's scope. So you could theoretically have the Amiga as a device in the firewire chain ;-) and the Amiga would just have a nice, fast CPU. I know this whole idea isn't practical or, really, even sane :-) But we're all insane here, so who cares?!
We've all long since gone past the point of being practical. I mean really, how many of us here bought a GBA1000 and are fully intent on soldering it together, all 476 components or however many it has, so we can have the fastest Amiga 1000 in town?
I'd buy a microITX PeeCee and stick it next to a 1200 in a heartbeat if I could run those old Haujobb demos twice as fast as I could with a Cyberstorm in a 4000.
I've tried UAE but just couldn't ever get it to run the old hardware banging AGA demos right, if at all. But that was a few years ago, so is the luck better now with them?