The graphics cards will support most any resolution that the monitor presents.
It is the individual games that determine the resolutions that it can run.
I would imagine that you would have no real problem with any game.
If the game does not support 21x9 it will almost certainly support a resolution that will work on your monitor.
As to performance, it will be similar to conventional 1080P.
And... keep your current monitor as a side monitor; it will not impact gaming on the main monitor.
Some games are graphics limited, and some are cpu limited.
To forecast which, you can run these tests:
) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.
You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.