Removing the pre-cat will free up around 10bhp on the VXR. Full exhaust is about 18bhp. CDTI airbox is a decent mod, but doesn't really give a bhp increase only really removes the flat spot at 4500 rpm.
First thing I'd tell him to do is replace the gear box oil with a proper oil, rather than the power steering fluid VX put in there as standard.
Exterior wise, not many people actually do much to them, other than carbon wrapping a few bits, or fitting bonnet vents.