Hi guys

I have just bought a 1997 wrx for some fun. Has bigger turbo, 500hp fuel pump, 550cc injectors, new coil pack, bigger intercooler, unichip, so i am told. Didnt really give it much when test driving. Since getting it home have give it a bit of workout and it splutters hard in 3rd 4th 5th, when under max boost. Also apparently has had plugs done recently. Have read so many threads i am confused as to what this problem is, any ideas? I have also found an exhaust leak not sure if it is before or after turbo. Help!