Brief update: I got the TSA replaced by the dealer two weeks ago (foc) which fixed the major problem. As far as I may tell after some rides shifting up is much smoother now, but there is actually some more delay regarding rev control by the unit - at least is feels as if the adjustment between motor and gear takes longer.
Shifting from 5th to 6th is now much easier at all rev rates, yet not perfectly smooth. Sometimes it works fine, some others I still have to release the throttle slightly to succeed.