Hook up a boost gauge even if just temporarily. The turbo should be blowing for all it's worth before 30 mph. You should feel the turbo involved around 2000 rpm and as much as the hand of God as you're going to feel by 2500 rpm.

Don't sweat the ALDA question. It goes on as easily as it comes off. Take some good acceleration time measurements as you have the car, pull the ALDA, repeat all the measurements. If there's a big difference, you'll know where to focus your diagnostic efforts. If there's no big difference, you'll know where not to focus your diagnostic efforts. If that sounds like too much work, disconnect the line from the intake manifold to the ALDA and check performance. Report back your findings.

IMO, the ALDA is the difference between fast and faster, not slow and fast.

Sixto
87 300D