I've tried different scenarios now.
Even though windows tells me about a gpu-usage about 1-2% for the vega64 it renders every scene i tested about 2-3 times faster than the gtx980 with a usage of 70-98%.
Sadly it does not make a big difference, if i render on both gpus +cpu. in the last scene i tried a testrender with 100 iterations: it just brought down the rendertime from 1:19min to 1:11min.
Thanks for your great work so far - i can't wait for further versions of rpr!