Im gonna make this short rob Your existing oem pipe diameter is 1 3/4". The available and most suitable upgrade sizes for the EP N/A are 2" and 2.25". I use 2" but a lot of people will recommend 2.25". I use the smaller because of the slight understatement in power the car has. A larger increase in pipe size will give you slight losses in torque the larger you go if you dont have the power or torque to maximise it. i.e on my 4efe the 2" system lost me lots of torque, I added the 5efe and everything was evened out. Right now its sexual. The factory exhaust design is maximised for good early low down torque and economy. Make sure to never EVER get a pipe bent system. i.e powerflow or similar. I have one and its crap. This is due to them pipe bending the angles in the exhaust thus making the bends smaller diameter at the bends restricting flow. You want a mandrel bent system where they use mandrels put inside the exhaust pipe when bending which are the exact same size as the exhaust pipe diameter thus you will get even sizes at the bends to optimise even air flow. This would be a more professional approach (mandrel bent exhaust system) and may cost a tad more but will give you peace of mind. A removable decat is optional. Apply cat back on for mot times if needed. Mine passes with full decat from manifold back. i.e nothing... Also there are off the shelf solutions such as the Blitz Realize systems or a jdm n/a system from yahoo auctions but they will be £500+ easily. As for noise Rob, a 4" jap can is always going to be noisy and thats the sort of thing I like, something 3.5" would quiet things down, i.e I have a quiet remus for example. Sparky should be able to give you some insight too as he recently had a system made, maybe he can tell us what he had done. My 2p Joz