Power loss might be the difference between a nackered old one and a new one? Clogged pipes etc.
I'd remove the turbo inlet pipes, intercooler and plenum and clean all the ****e out. The new turbo might be being strangled by the old ones problems ...
Might also be worth checking the fuelling and injectors/pump etc ... new stuff on old might need to be set up again to match what's there.
You can clean it all out easily enough, but from what I've read the best way to tune a TD5 is to chip it. Having said that, I'd clean it all out, service it, especially fuel pipes and filters and basically just fettle it a bit. Have a search on the forum there are a few articles on it.
If it's strangled at 3k then there's a problem that needs sorting first, hence why I suggested the service etc. I wouldn't recommend chipping anything other than on a properly working vehicle.