I bought a 40mHz FPU for my BlizzardIV030 (50mHz) a few years back. At the store they told me there would not be a problem. True, the FPU works ok for about half an hour after which FPU related apps start failing. I guess heat is the main problem. My A1200 is a standard desktop one and has very limited ventilation (particularly in the trapdoor area even with the lid off).
I guess you are ok with a 40mHz FPU provided you have cooling sorted or your 1200 is towered.
