I'm glad you like the idea
Sorry, I have no idea what kind of fp performance is needed.
From "top" i can see that guitarix standalone consumes about 12-15 % cpu on a core i5-2405S (running all cores at 1,6Ghz, ondemand scheduler). What proportion of that is fpu i can't tell without further digging.
With the engine disabled ,the gui alone consumes about 2-3%.
Also, jackd and the irq-thread for my firewire soundcard consumes 25% and 15% respectively. The firewire would of course be out of the equation for Raspberry Pi, but jackd would still be needed (most likely running with the alsa backend). Not sure how much of that would be fp in jackd's case, i think at least it does some sampling rate conversion if needed.
Also, the above numbers are from running with a sampling rate of 96 KHz, 24 bit resolution, which is probably way overkill. For live guitar sound i think 44 KHz (or maybe even less, most guitars are not that wide bandwith) and 16 bit is good enough.
Been thinking a bit more about this, as long as the fp performance is adequate, it should be at least doable. Custom hardware is of course needed:
Hi-Z preamp + ADC/codec (plugged directly to the i/o on the board)
Basic controls (input gain, input overload led, preset switching + display, and output volume)
Possibly an additional headphone output for in-ear monitoring.
Those things should be possible to squeeze into a quite small box, say the size of a guitar tuner.
You could possibly interface the thing with a usb/midi foot switch, but i think i'm getting ahead of myself...
Some custom software would be needed too, but i'll get to that in a later post.