"There are also fears that valve seats can get sooted up as they only handle air and not a nice slippery petrol/air mix"
It is true inlet valves and inlet tracts get encrusted but the reason is because of PCV crankcase oil fumes. Mine will never get sooty since I've plugged the PVC pipe inlet in the inlet manifold and run the breather to a catch tank. I have advocated this several times for GDI engines and use the same system in my BMW 340i.
