So, as a matter of fact, all of this "unstable handling/steering" geometry issues started AFTER I replaced my steering rack. When I bought the car, it drove just fine, but after experiencing some play on my steering rack, I bought a new aftermarket TRW. TRW was a good choice I thought. After the first alignment it didn't feel alright, then someone recommended me to use slightly more agressive alignment specs, also drove like poop. Then we're where we are now. So there are 3 people in this entire country which actually have experience with alignments on E46's, 2 of them build cars for the track and even have customers coming from foreign countries just to get their alignments and performance mods done. I explained my situation and one of them said he had an E46 with a similar problem, only worse. Despite having "good" alignment specs, and done a complete suspension (bushings) overhaul, his car still drove like ****. As a last resort he replaced the steering rack, and all problems were solved.
I have no idea what could go wrong with a steering rack other than play or leaks. But then again: my issues DID start after replacing my rack, but it's a brand new one. Could it be that TRW's have maybe a slightly different "setting"? I still have a spare one from a 330iA I stripped about a year ago, but that's a lot of work, just to see if the problem is resolved afterwards, and having to pay for yet another alignment.