IIRC the twin pipe system was designed to allow better gas flow and release a few extra horses, but the best sytem is the 3.9 EFi, biggest of the lot, with good gas flow if pre-cat and not much between that and my Double S sports pipe.
For a cheapy system ours had a TDI centre box (straight through unlike the EFi one) and no back boc, just the pipe, sounded great when the engine was on-song but terrible when it started to do valves.