Power Boost valves increase the amount of fuel your car takes, they do not by any means increase BHP. I found that on a 1.8 8v MK2 Astra SRi it made the pick up quicker, but on 16v engines it's not as noticable. seens as yours would be an 8v engine, it could possibly be of some use, but it varies so much from car to car!
Exactly. It all depends on whether your car is overfuelling or underfuelling. An adjustable fuel pressure regulator (powerboost valve minus the naff name) is useful, but best adjusted on a rolling road.