what will a Full DE-cat system give my 94 wrx???

HKS filter, Magnex decat down pipe , Tanabe muffler + mid pipe???

just want to know how much BHP i have gained from these mods

 

All cars are different, especially the WRX's, you dont say if your car is a wagon or a saloon, what fuel you run it on as this will effect it, do you have a knocklink (recommended to see whats going on under the bonnet, doesnt give you any performance but could save you the price of a new engine )
The only real way to check what BHP your car is giving is to put her on a rolling road, otherwise it will just be an estimate.

Tony