Soooo. For whatever reason none of my USB sticks are successful in getting Sierra to install. But I did install fresh HS on a spinning disk..
One notable difference is that apparently native driver was able to give me 4K from the get-go on one screen. Installing web driver gave me both screens and 2K mode. Now trying to get a benchmark tool name, that I used to measure stuff in another install. _Seems_ a bit faster, although let's say resizing a heavy browser window is still very very sluggish.
I'll try to get myself a stock of new USB sticks tomorrow and retry with Sierra again.. Man, this is frustrating.
Burn a disc. USB installer is not a good option on cMP, lots of user never success.