Hawkeye 2.5 litre STi from 2006 - 2007 ....

In std form they are 265bhp give or take and have boost at about 1 bar peak dropping to about 0.8 bar at the top end.

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 but not essential - the backbox just adds some burble to the quiet hawkeye exhaust.

With a custom remap on the standard car typically this would get the car to around 310-320bhp with significant torque improvements.

2. Sports cat in downpipe - plus 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 highly recommended.

It is also advisable to change to NGK 7 grade spark plugs at your next service.

This would get the car to about 340 - 350bhp 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 and really sets the 2.5 free. A remap is needed to bring it under control and stop overboosting that will happen enormously with the 2.5 engine. The remap will bring boost in as smooth as possible and most likely get full (and higher ) boost at around 2700 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 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 350-360bhp 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. I remember this stage well on my own and it is a fantastic car to drive on the road.

A walbro 255 fuel pump is essential.
It is also advisable to change to NGK 7 grade spark plugs at your next service.

4. 350bhp - 400bhp

You may be one of the lucky ones and have a car that wants to produce power with the std turbo - the VF43 on the std car can get to around 360-370bhp tops but if you want to get to up closer to 400bhp 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, gearbox and clutch (just) can cope with this power.

It is adviseable to fit a larger diameter MAF housing by way of a good quality cold air induction kit.

5. Getting to over 400bhp

The std EJ257 STi engine has been taken over 400bhp but really it is adviseable to swap the cast pistons for some forged pistons at this stage.

Conrods would also be a worthwhile step if planning on revving higher than the current 6900 limit.

On top of this, there are clutch options, fmic, headers, up-pipe and lots of other items to consider such as extra fueling. It can be very expensive step going to over 400 bhp on the 2.5 and if for the road I would say that it is not worth doing it.

If you do decide that 400+ is 'needed' (lol) get lots of advice beforehand / look at similar cars that run over that level and decide whether you really want to chase the big numbers as it becomes a diminishing return for the money outlayed. In most ways it is just as expensive going for over 500 as it is over 400 so you may as well make that step I did - it is fun for sure 8)

... hope that is useful.