overvolting

I have met Brits who speak worse English than you do Markus, no worries there :P

If it works fine with 6 cells and a bigger motor, I can't see why it wouldn't work with the smaller motor - I would keep an eye on the amp draw though as the motor itself might draw more than the 45A peak at startup on 6 cells. Also, as you've guessed, there is a sizable chance of the motor exploding at 6 cells, but you don't know if you don't try, heh...

Ok. The ESC kan handle 80 A continuously and 100 A for a
short time, so I think that wont be a problem.

I guess that I´m just afraid that the speedcontroller suddenly
will turn into scrap, as my Sabertooth did... :(