Lightweight flywheel has got to be top of the list. Doesn't give more power but lets the engine spin up faster and makes it feel like its got ten extra horses in there just bursting to get out.... Should be around £100 or so plus the 50p Nelly's bound to charge for fitting it.
Personally I think I'd go for a big bore over the plain hi-comp pistons. It'll cost more for the rebore and replating but the 944 pistons are higher compression as standard as well as being larger. As our colonial cousins are fond of pointing out, "There ain't no substitute for cubic inches".
Then you could get some porting work done and a set of Keihins would be nice too. How 'bout biigger valves as well. Lightweight clutch, although it won't be as noticeable as the flywheel 'cos it doesn't rotate as fast.
Skies the limit with the 2-valve and since you're flush could I borrow a few quid please....?