I got reboots again with 10.12.5 and AGDPFix 1.3, so no difference there. I'm trying now with the iMac14,2 system definition. I do have different CPU states now, but after regenerating my SSDT the full range still seems to be there, so I guess that's okay.
Update: it still reboots. 10.12.5 and the Pascal card just does not work for me.