That is like asking how long is a piece of string! It all depends on how much power you are after and how much you can spend . As others have said ,cylinder head up to group A specification, BD 12 or 14 inlet cam, maybe T38 turbo with billet compressor wheel, Siemens 83lb injectors , L8 ecu , custom chip and wasted spark ignition to name just some of the modifications you can make.
The higher the specc you want bear in mind other parts will also need upgrading.
Best of luck.