I don't think you need your turbo to get the RPMs over 2000, the engine should be able to do that without it. This sounds more basic, like a fuel problem.

The engine will run much higher than 2k. He needs at least 3k to highway cruise at what he quotes.

The alda circuit on the injection pump isolated from the boost effect will present what feels like not only poor power but inferior acceleration. He should normally feel the transition point a little when the boost comes up and the alda starts letting more fuel in to burn. This starts to really occur at about 2k rpm.
Instead he is performance flat in that area and is struggling to get by it. What is painful is he has had this problem so long yet it is so common an issue. You can have boost till the cats come home. With inadaquate additional fuel from the alda control it is absolutly pointless to have it..

Now to be fair the issue could be something else. This area is pretty straightforward to check out and in many cases costs nothing to do. So it moves to the head of any list automatically. Many,many past owners have disposed of these cars for this particular issue.