just to report back:

my issues with temperatures are solved now! replaced the tim under the r3e stock sinks and this brought ioh/ich idle temps to about 60C! remember before this temps were rising quickly and reached 100C within minutes, thus the board simply switched off!

replacing the tim with some ocz freeze dropped temps by more than 40C! (expect any tim will do the same - asus stock tim simply is crap) heatsink is still fairly hot. ir thermometer showed about 58C on top of heatsink.

left the stock pad under the vrm sink. would not recommend replacing this as regular tim might not be able to bridge possible gaps between the sink and the mosfets.

reseated cpu cooler too. at first power on, cpu temp again rose slowly up to about 65C. but after the next powercycle cpu temp stayed below 47C. no idea why it ramped up like this at first power on. did the same at my first mount? anyway, as this the 775 cooler is a temporary solution i will ignore this for now and check how this behaves under water.

conclusion: replace stock tim on r3e!