I'm guessing it will be around 400-600Mhz, which should still be enough for AmigaOS.
Well, OS 4.1 actually runs quite nicely on my 240MHz BlizzardPPC. The biggest bottlenecks are, aside from the total lack of L2 cache, the fact I'm using the original A1200 ports for everything (motherboard IDE, PCMCIA network) and slow 60ns RAM (compared to anything more recent). Of course, I'm also using an RTG card which helps enormously over AGA. So yes, it should be sufficient.