yea, the step up from ko3 frame to ko4 is fairly big in terms of parts you need, obviously you've already got the MAF Jamie, but you'd need bigger injectors again too, plus all new oil and water lines as well.
Plus the ko4 downpipes don't mate up directly to a golf catback system either, so it's all hasstle, but well worth it!
I took my AGU to a typical st2 spec and left it like that, and spent a few years developing the rest of the car, then when i reached a point where the chassis seemed ready, I went straight to the ko4 hybrid and havn't looked back!
i can't imagine it without the power now, it's just so addictive!