Hi, now I am making progress.
Now I fixed my leaks, replaced the boost valve and the blown exhaust manifold seals. Now car runs perfect, Lambda-values are okay, no knocking under high loads, boost control works perfect.
At this starting point I installed the modded file on ME7.5 with the 180HP TT-mapping. It runs perfect. Car drives much better and no problems under high load. no kocking and it doesn't go lean. boost is stable in MWB 115
So much power in a small car like this is perfect. :-)
Thanks for your help everybody!
Yogi