I only run an aftermarket exhaust cam and I made 522 bhp , it's the old tale again about inlet manifolds and exhaust manifolds etc etc etc
You could probably gain a fair few bhp by adding custom cams but it all depends on your budget , cheap then stick with original cam but if you want to chuck a few hundred quid then the choice is yours.
Look at Paul Johnson zetec turbo made another 70 bhp with cams and a different turbo