I used to have an A5 35 tfsi and noticed something similar:
When I wanted to overtake I would press the throttle, it would slip into neutral, rev like crazy, then eventually pick a gear to allow me to overtake. assuming of course by that time it was still safe to do so. The accelleration was decent enough when it finally got around to it though
I just put it down to it being a detuned 40 tfsi