In std form they are 220bhp give or take and have boost at about 0.9 bar. The boost profile in the std map is pretty tame and doesnt come in til 3500rpm assuming the car is std and starts to tail off from this low level at around 4500rpm.

1. Keep the car std but just remap for more oomph and better fuel economy.
A full custom remap will bring boost in much earlier and hold higher boost levels for longer but the map will be geared to running much more efficiently ( which is also more economically) as it wont be possible to push too much boost with both cats in place. Typically a Green/K&N/Cosworth air filter and a backbox are fitted at this stage.

This would get the car to about 250-260bhp

2. Sports cat in downpipe - remove centre cat
A full custom remap will bring boost in much earlier due to the lower back pressure and hold much higher boost levels. The remap will also aim to get it running much more efficiently ( which is also more economically).
Typically a Green/K&N/Cosworth air filter is fitted at this stage (on top of the sports cat downpipe and exhaust system).

This would get the car to about 270bhp but a huge boost in torque figures in the mid range.

3. Fully Decat Exhaust
A remap is needed to bring it under control and stop overboosting. The remap will bring boost in as smooth as possible and most likely get full boost at around 2500 rpm and hold higher boost right through the rev range. The remap will also aim to get it running much more efficiently ( which is also more economically).
Typically a Green/K&N/Cosworth air filter is fitted at this stage (on top of the decat downpipe and exhaust system).

This would get the car to about 280-290bhp but a huge boost in torque figures in the mid range to around 360 lb/ft and transform the way the car drives on the road. A real STi Killer this one !!

A walbro 255 fuel pump is advisable but not 100% essential.

4. Getting to over 300bhp
A lot of people want to do this, it is a nice bhp figure for the pub afterall. So .. how ? The std TD04 turbo will not do it unless you are on a dyno that reads very optimistically.

It is very easy to do on the 2.5 wrx with only a turbo change really needed to do it. The ideal is the VF43 off its bigger brother STi version.

There are many many choices of turbo for this VF30, VF34, VF35, VF43, VF48, TD05 16G, TD05 18G etc etc but all of these will need a couple of other changes to the car before hitting the mark safely.
- Walbro 255 pump
- the hawkeye WRX TMIC is very good so you wouldn't really gain anything from upgrading to the STi TMIC or another top mount.
- there is no need to change the up-pipe as the 2006/2007 hawkeye already has a decated up-pipe.

It is also advisable to change to NGK PFR7B spark plugs.

There is a lot of info around about the different turbos and what they are capable of - the VF43 from the hawkeye STi makes the best upgrade in my opinion at the moment and should hit a nice safe 340-350bhp. The VF30 makes a great upgrade too. This is pretty much an ideal level for the WRX keeping it within the capabilities of the engine, gearbox and clutch.

Some other info:
- a vent to atmosphere dump valve will not improve the performance but if you must fit one for the sound then stick to a good make - forge have always been good from what I have seen. (Baileys are less so)
- an induction kit will most likely not improve performance as the std airbox is very good. If you do want more induction noise then go for a cold air induction kit such as the K&N typhoon where the filter is located away from engine bay heat.