the exhaust probably won't give it much more power. make sure that all your electrical stuff is good, and everything is working how it should be before putting an exhaust on. would be worth getting it taken to a place with a gas analyser as well because mine is running lean (which i found out through the gas analyser) and that is why it lacks power.
when you get the exhaust i would go with a 2" system with two perforated tube resonators and a perforated tube straight through muffler with a 2" tip. I am going to opt for custom headers/extractors and an aftermarket cat, but how much detail you want to go into is up to you. I would like to point out that the idea is to maintain gas velocity but be free flowing at the same time, and too big an exhaust will lose velocity.
L series exhaust
thanx mate, ive done some more calculations and taken a further look at my car and have decided to go with a standard exhaust for now simply to replace the pile of metal thats there atm and leaking nicely. i picked it up for 900 so i knew there were gonna be some things fall off down the track, such as wheel bearings...and suspension...and the need for a serious tune up in general. did i mention the clutch is dieing too...but as i said, for 900 and 210000 on the clock, im quite happy to spend a few wads of cash to bring it back to life, anyone wanting to help is more than welcome to 
it could also do with a new paint job, someone keyed several areas of the car...
it could also do with a new paint job, someone keyed several areas of the car...