it looks like a tight fit and l dont wanna have to return it if it deosnt fit

In all honesty, unless you're going with a massively high boost, full-race setup, the stock recirc. valve is plenty good on a stock STi.. unless the car is running a rotated turbo, or is full race/ rally.. leave it as is.

Ohhh, but he wants that sweet pssshhhh sound to go vroom, vroom. First upgrade the turbo, injectors, tune the ecu, a better than stock clutch. Then upgrade the intercooler, the brakes, and be fast.

Consult your nearest performance shop. Those performance goons will be more happy to give you the best of the best suitable for your taste.
Actually, all blow off valve are the same, they can pre-set to your needs. Except, what noise you are looking for.
Whooooosh !

If it is the sound you are after then try a short ram intake or remove the intake silencer in the fender and you will hear the sound pretty good. There is no performance advantage to a atmospheric BOV. In fact over time it will cause damage to your motor.

The stock BPV can hold up to 20Psi of boost safely so unless you upgraded your turbo you dont really need it but if you feel the need to hear that blow off at least get a 50/50 recirculating one

leave the stock recirc valve, if you really need the blow off sound, invest in a snorkus delete pipe. Or at least get a 50/50 like a GFB, stock valve can take up to around 350-400hp+. Do a search on NASIOC, there is a lot of mixed input about problems with running full VTA. Kinda hit and miss, some get by, others can not, but you will definately run rich without a tune.