druck: My understanding is that it's not pin compatible, and not entirely software compatible (although I don't know if RISC OS uses the features for which there are differences.) I agree with the doubt that the market isn't big enough to warrant creating an upgrade exclusively for RISC OS users.
However, I suspect if building a machine from scratch, it should be pretty easy* to build a machine that's 2x times faster than the Iyonix. The parts are available. What's missing is money, and unless Castle can find a client who wants such a device, I don't see it happening. And there isn't a big market for such motherboards even outside the RISC OS world. Advantage 6 have been sailing on the back of the embedded business: a large ATX board such as the Iyonix's doesn't fit in there.