Finally got the nvme to boot.
In short, the solution is to boot (with legacy BIOS) to a usb drive that contains another bootloader (i use clover with nvme driver and windows efi bootloader). The windows bootloader needs an entry to boot from the nvme, this can be done quite conveniently with a program called bootice.
Anyway, the system now boots to nvme and deliveres disk drive speeds up to 814MB/s. The nvme could do more but the MB has pci-e 2.0 which is the bottleneck.