You may want to try the APEXi exhaust system. The new series is the N1 exhaust.
The dunk series is also ok.
It has one muffler and the piping is straight through without any bullets. The down pipe you may consider 2.5 inches or 3 inches. But with the 3 inches down pipe your boost amy not be stable on high / top speed.

There are so many aftermarket exhausts out there, is there one that provides the best performance or are they all of a muchness.

Also, does an induction kit like the £99 pipercross claiming to give +8bhp do a better job than a normal aftermarket filter.

Well David, I would reccomend a Scooby Sport Exhaust and RamAir Filter not a induction kit. Speak to Pete @ Scooby Sport for more advice. Exhaust and Filter costs £370 posted to your door. My exhaust is on order and will arrive 7th December - watch this space for some pics!!!

With regards Air filters be aware that RAMAir will be dropped by Prodrive next year as they have gone bust and been rebought. The 555 Rally team use Pipercross who will be supplying Prodrive next year.Barrets motorsport have also been developing filters for Impreza WRX's with Pipercross and will announce these next year.

I had a Scorpion exhaust fitted at the weekend and i'm very happy. With a 4 inch tailpipe i was a bit worried it might be a bit OTT but once on the car it blends in nicely. The sound isn't intrusive in normal use but once you accelerate hard it changes to a hard edged roar. Response and mid-range urge have improved (probably aided by the Ramair filter). An ECU reset this week might possibly improve things further.

How does changing the exhaust affect the Subaru warranty?

Imran, I can't tell you how the Warranty is affected for replacing the whole exhaust system, but I can tell you that I called about fitting a back box. I got a number from my local dealer for some type of HQ who are responsible for the importing of Subura, Ssangyong and one other that I can't remember. I spoke to a guy in the warranty department and he assured me that fitting a back box, would not affect my warranty on the rest of the car. In fact, he was more concerned with the Ramair filter, which I found rather bizarre. The general message was that providing you are fitting a part manufactured to at least the same standard and that part did not directly cause another part to fail, then all is OK. Hum. Although I reckon 'chipping' is definitely a no no!

Unless of course said chip happens to be supplied and fitted by a certain Mr David Richards (figuratively speaking !) and which just so happens, I am told, to have been deveoped by another chip shop (S**er**ips)

I always thought highly of Superchips. It doesn't seem like them to produce a chip that makes the car slower & less powerfull...

Scooby sport back box.
It does not have a ridiculously large tailpipe, sounds brilliant,is not obtrusive and just take a look at the quality. It is manufactured using stainless steel and is a thicker guage than most other aftermarket exhausts. Infact it's a work of art. Fitting it yourself takes no more than 20 minutes and it does wonders for the drivability. I simply cannot recommend Pete Croney's help, enthusiasm and products highly enough.
I have heard that the induction kits are too noisy and my views on the ramair filter are purely my own and have been documented elsewhere on the BBS.
Speak to Pete Croney, buy an exhaust and drive everywhere with a silly grin and the window down.