Update:
Massive step forward for me... ALT+TAB in and out is fixing the FPS issue.
So, outstanding issues are:
- The display is cut off at the bottom when in the inventory overlay.
- SLI not working (currently maxing out a single card)
The display being cut off is a game issue whereby the menu files are not designed for 21:9
There is a fix on nexus that replaces the inventory to work with ultra widescreen
http://www.nexusmods.com/skyrimspecialedition/mods/1721/?