GOOD News:
Well, by changing from 3538L to 3534F my system got power all of a sudden! Very clean recording results with 10ms, good ones with 5ms.
(1 stereo track 32bit)
CPU-Usage not over 40% while recording
20% while playing - that's what I call sensible
Multitracking not tested so far, but I can feel it, performance doubled!
3534f (+ ProFx v3.08) makes the difference for me.