I did like you suggested and flashed the RC7 firmware.
I also printed the heavy duty feet from thingiverse (http://www.thingiverse.com/thing:1459025).
Maybee i will also be adding the more rigid cornerblocks.
The more rigid feet help to counteract the judder. I think the whole setup was just too loose.
I also found that estlcam was not setup correctly. This caused the slow movement. When changing the settings i mistakenly set the feedrate (in the output options) to mm/s, while you clearly state it should be in mm/min. After i corrected this, the speed seems to be correct.
The fix for the hot Z-stepper was that the amperage was just to high. Fixed that aswell.
Thanks for the help 🙂