From extensive research I found WinUAE only needed Pentium III 600mhz for OCS/ECS and Superstardust AGA needed 850mhz. These were laptops running Win XP SP1 in 2007ish.
I'm not sure how fast the Pi is.
my openpandora emulates the Amiga quite decently (not sure what its like at default 600mhz I'm overclocked to 800mhz), the pi runs at 700mhz as standard and is also overclockable so I imagine if the pandora version of uae was ported to pi (different ARM version) it would be quite usable.