Any decent Custom builder like C&C Custom Stainless in Hillington or Meercat will build you a system to your requirements, with the number of resonators fitted that can be squeezed in/baffled back box etc, with a minimal loss of power. Support local builders by buying local - that way you get local service too, not that there is any good reason other than this not to buy from the likes of H&S.
2 1/2 inch system is good for back pressure unless planning to go above the 340/350 BHP bracket (as Mark said above), when you should get a 3 inch system. Also depends on the size of your turbo.
Brian []