get some dish washing liquid, mix it with water, but a fairly thick mixture. use a brush and put it all over your intake piping. You *should* see air bubbles if there's any leaks.
Obviously, check your actuator hoses as per above and make sure they don't have any leaks - but usually, if there's a problem there you're overboosting, not underboosting. You could try disconnecting it, if that fixes your problem you might have a faulty solenoid (does that happen, never seen it??)
Make sure your timing is right, you can get a big dead spot if it's wrong.
Usually, if you don't have enough fuel or it's not firing properly you get an abrupt kinda cut out, so that's probably not it.
It could also be a faulty wastegate actuator, but that is usually only apparent high up top and looks like a boost drop off, so that's prob not it either.
Any kind of exhaust side gasket leak isn't going to cause that much of a power issue, plus you'd be able to hear and smell it in the engine bay, so that's probably not it either.
The thing is, if you've just put your motor in a variety of vacuum hoses, either missing or connected the wrong way can cause the problem you're describing, so it's a matter of trouble shooting it, and having a close look at GTi-Rs that actually work. You might also find this useful:
http://www.gtiroz.net/workshop/
