VRPC is, of course, a problem for Castle's pricing policy.
Having said that, isn't there already a faster Xscale (800mhz IIRC), so they could surely bring that in at say, £1100 with a big HD and high RAM, and bring down the 600mhz to £800.
Alternatively, they should go back to the self build option. An Iyonix motherboard for (£400 about the price of a high-end PC motherboard) would tempt many users and Castle could issue an easy 'no support' disclaimer. For me, and I'm sure many other users, that would be effectively a RISC OS 5 32bit machine for £400 (I have a 40gb IDE drive from a G4, a huge ATX case with PSU and an 8x sony CDROM currently going to waste).