I mean I can't honestly see much difference between RO 4.02 (which I am happy enough with) and RO 5.XX.
Where there *are* differences they are generally in RO 5.XX's favour (like allowing for ultra-large drives, using UDMA at many times the speed of the old IDE on the RPC). Support for the xScale means that we're not likely to be left high and dry without a processor anytime soon. Also with a degree of hardware abstraction it means "retargeting" the platform for other uses becomes a lot easier (assuring the platforms future).
Now granted Castle needed to put a HAL in there to allow the Iyonix to run (while I am not a major fan of HAL's - they can sometimes hit performance - Castle seem to have got the balance right). The machine I saw in Guildford last year seemed quite fast.
You ended by saying "you can't say an Iyonix is a good replacement for a RISC PC", that I would dispute - but I'd also add "the Iyonix is the *only* replacement for the RISC PC" - and that my friend is the reality of the situation.