For a start a BTB system is designed for serious performance producing engines and on a standard or mildly tuned engine your probally lose power so thats no help at all, like I stated in my 'Big Exhaust thread' bigger isnt always better - a common mistake people make.
For mods right up to changing Cam/s then a normal off the shelf sports exhaust is fine and depending on brand will perform well, you only really want to look a BTB system and manifold if your running say throttle bodies as 'itsafastworld'.
Now any of the 4branchs on the market will see a good gain over standard more so on the R than the S as the S already has a Factory 4-2-1 but still gains can be seen with a aftermarket 4branch.
If your looking to for better performance look into decats remember the cat has to go back for MoT time, and maybe single box ie just the back box and a straight through centre section.
Have a read of my 'Big exhaust thread' click on the link under my signature thats labled Big exhaust thread
