The BPV is the device that opens when you have let off the throttle and the positive pressure in the intake manifold needs to go somewhere (and not back out the compressor wheel of the turbo, causing damage). The WG (wastegate) is the actuated valve the controls how much exhaust is diverted around the turbo. The exhaust is what makes the turbo spool, so by controlling how much exhaust goes through it, we control the amount of boost the turbo makes.
It should be noted that the computer controls the WG on our cars (which is one of the biggest factors that makes the tunes on our cars worth so much), which is why I suggested that the tuners take a look before anything else. If the BPV was leaking (or any other boost leak) you would have a magnificent loss of power and almost assured CEL (check-engine light) because the computer wouldn't be able to regulate the fuel (the meter would be seeing a lot more air coming in than was making it to the cylinders and you would run obscenely rich).
