I know you could get PCIe graphics cards to work with 8 lanes, but did you ever actually use such a crippled device? They are slow. Really slow. And in a computer, you would need at least four lanes for the other expansion slots, wich leaves us with only four lanes for the graphics card.
For me the biggest issue with the current Iyonix has always been the non-availability of an AGP port, because PCI graphics cards always lagged years behind. Now the world has moved back to a unified expansion interface for all cards, and we get a chip that again would severely limit the graphics capabilities of a RISC OS computer based on it.