Ok. Firstly apologies if this is unhelpful to others posting here.
Those for whom the Alt+ Ent issue is still a sorely missing feature (vr mainly or multiscreen) it is probably of no use.
After 6 hrs of fiddling I resolved my individual issue.
In Nvidia control panel I set max frame rate off (had been set 60 to match monitor)
I set Vsync to ON (had been set to control by 3d application option - thinking DCS would take care of it)
All other nvidia settings are at default.
In DCS vsync is ticked, Full Screen is ticked.(my defaults anyway)
This simple change fixed performance problems in both graphic fidelity and returned my GPU load back to below 50%.
Go figure.
Thanks to all for the input. I will mark this resolved (for me) I hope others can find solutions as well.
Cheers Big Newy.
