Have you ever considered a custom system?
I've had anumber of systems fitted by a small man man company in Crasyhill just outside Brillericay called 4extreme performance.
Here are the ones he's done for me
Saxo cat back straight pipework into a custom groupN style 2" taipiped back box £350 but had my manifold changed at the same time which he fitted
Back box with a 3" tailpipe on my 740 Volvo, he also removed my centre box as it was a very quiet back box. £220
Fitted a supplied back box for my Gen2 Honda Prelude EX £90
Cat back straight pipework to dual custom 3" cans on my 2.0 16v Cougar, very loud, bubbled, popped and farted like a turbo

£300
I can dig out his number if your interested