Again, a common question that I get asked about what should be done and what shouldn't etc so I will aim to do these for each significant model type.

2.0 litre STi from 2001 - 2005 ....

In std form they are 265bhp give or take and have boost at about 1.2 bar but this varies according to model. The boost profile in the std map is pretty tame and doesnt come in til 3500rpm assuming the car is std.

so... choices .. (all based on using vpower)

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.

With a simple remap on the standard car typically this would get the car to about 290bhp (blob eyes may make slightly more)

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). A walbro 255 fuel pump is essential.

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

This would get the car to about 320-340bhp but a huge boost in torque figures in the mid range.

3. Fully Decat Exhaust
The low back pressure from removing all the cats reveals the true characteristics of the boxer engine. 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 (and higher ) boost at around 3300 rpm (depending on turbo surge point) 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 and possibly a 3 port solenoid are fitted at this stage (on top of the decat downpipe and exhaust system).

This would get the car to about 330-350bhp but I have seen them up to 365bhp on the std turbo but a huge boost in torque figures in the mid range and transform the way the car drives on the road.

A walbro 255 fuel pump is essential.
It is also advisable to change to NGK PFR7B spark plugs.

4. 350bhp - 400bhp

You may be one of the lucky ones and have a car that wants to produce power with the std turbo but if you want to gtee over 350 there are a number of turbo options and rather than list them all here - gimme a shout and see which of the many turbos around could fit your requirements. There are lots of suppliers of turbos and they are changing all the time.

The car's engine and gearbox can cope with this power well.

5. Getting to over 400bhp

The std STi engine has been proven to up to 450bhp but it can be an expensive step going from 350 to 450 so again get lots of advice beforehand and decide whether you really want to chase the big numbers as it becomes a diminishing return for the money outlayed.

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)
- a FMIC is not needed at this stage but can be a good improvement on the STi tmic when above 350 - the sti tmic can support up to 400 ish bhp though for road use.
- 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. It may be that the time you 'need' to change to an induction kit then a FMIC may be being fitted so it would need to be one that works well with your chosen fmic.

Anyway ... hope that is useful.