Upgrade from Citrix XenServer 6.5 > Citrix XenServer 7.6 > XCP-ng 8.0.0 with prefinal .iso went smooth without any problem on a 3 box pool. Moved to the "new" partition scheme from XS6.5 -> XS7.6 and used the prepare_host_upgrade.py (rolling pool) method. Had no issues migrating the existing VM's upward.
Perhaps it would've been possible to go from XS6.5 straight to XCP-ng 8.0.0 (after applying hotfix RPU002) but since this is a production cluster I didn't want to make it too risky (yes, I know this is not quite final yet). 
//edit: Forgot to mention these are Dell PowerEdge R720s on Intel E5 2660's using FC storage.
Thanks XCP-ng team! 