you could do with a decent induction kit before an exhaust system. a straight through or engine back will be the best performance as there is no restrictive cat in teh way but they come MOT time you will need one.
i haven't researched into exhausts that much as im not looking for one right now but i do know that alot of the race spec exhs liek buddy club and spoon exhuast are pretty loud. you can easily and relatively cheaply get one custome made and get the right bore for performance and get the right sound through choosing how many boxes you want.
manifolds: 4-2-1 4-1..... what does it mean, why are they different!
4-2-1 gives lower peak hp figures but improves low end torque, where you need it, also gives gains across the whole rev range. better driveability when going normally.
4-1 loses low end torque but improves peak hp at the top end. best used in race applications as you always are revving to the limit.
de-cat/sports cat: catalyst is the main restriction. removing gives gains on nearly every car. removing increases emissions and normally a FAIL come MOT time. only work when hot (car produces more emissions when cold). sports cats work by reducing restrictions but still reducing emissions. are very expensive.
cat-back. these keep the catalyst and reduce restrictions on the rest of the exhaust.
power gains would be manifold de cat and cat back system.
manifold-choose 4-2-1 or 4-1. (i have 4-2-1)