JesseT I had thought of this but if i'd destroyed one of the lobes/lifter bases would it not be tapping like mad?
As you say at lower revs I am letting more air in but it's not just air it's fuel also, it's mixed before it enters the chamber so the AFR should still be ok, and as with all MFI RS Turbo's we run rich midrange anyway as the fuelling can only be set for full load and idle so it shouldnt be lean and detonating (IMO).
But as you say with good compression it seems less likely to be the head gasket anyway.