Know haldex can handle 400odd-500oddbhp (for how long before it fails in some way I have no idea)
Anything over 500odd if never seen in a mk4.
Also to the op. Enjoy trying to get 6-700bhp from it an 1800cc
Friend of mine has a 1800cc turbo engine car. For him to reach 700 bhp it has been board out to a 2.3 letre garret turbo and around 10+k of other work done to it....