Isn't a "power boost valve" just a fuel pressure regulator? - Personally I don't think you need to replace the stock FPR to achieve 300bhp.
The logic is this; generally it's making more power by increasing the fuel pressure so that you inject more fuel with each pulse, but you can achieve the same thing by uprating the injectors. More pressure = more chances of things going wrong (hoses coming off, injectors sticking because the solenoid can't open against the pressure, that kind of thing)... of course you need to remap (or at least fool the ECU) if you replace the injectors.
Has the fuel pump been uprated? That's probably the key thing you'll need if you want to start playing with the boost (and playing with the boost is going to be the easiest way to make the power you want); probably you're looking at about 1.0-1.2 bar of boost, but who knows every dyno gives a slightly different result anyway.
I'm running just over 1 bar, and I think I made 254bhp or something like that; admittedly I had an ignition problem, so maybe not a great example.
You might be able to achieve more as you've already got this valve thing fitted.
I find it a very confusing name when used on a turbocharged car because you've already got "boost". So "boost valve" could just as easily mean "dawes device" or "bleed valve".
EDIT: You got there first Mark; too much waffle from me again.