Thanks Pete and Thomas,
all that Lude2Envy stated regarding the other planes working without issues, including PMDG, A2A is valid for my case as well.
Thus I never assumed it is a FSUIPC issue and still believe it is not. Flaps and spoilers work just fine, which is an indication that it is software AS related. It can be, that they are operating with absolute numbers.
The commands in my case were sent directly to FSUIPC for calibration, tried it with reverse and without reverse thrust checked (and set). If the 'joystick calibration' in FSUIPC is set and calibrated, the throtles work 0-50. if this is reset and not set again, they are working in the range of 25-75. I guess it is the reading of Aerosoft that assumes as Pete said: -16/16. TQ6 has a resolution of -8k to +8k and this can be the reason why things get funky. However Saitek TPM has a resolution of -16/+16 and it does not change the matter.
I did try "send to FS as normal axis" and the problem is exactly the same. I get the half range and not more. I do not use p3d to process my axis and do not want to use it, but to eliminate all questions about it: no, it does not work the whole range either, only half.
I momentarily gave up on the issue and am using Spad.next for throttle assignments - there they work as intended, the whole range, at least to TOGA. Full power is still elusive but it may be systematic and connected to the plane's logic which I had no chance to get acquainted with yet.
cheers, Matt