Ultrawide Monitor - Clover Distortion

This falls under the category of First World Problem to be sure, but none the less.

I have the Acer x34 monitor which is a 21:9 aspect ratio.

When my bios (and also clover) pop up they are stretched wide as they were designed for 16:9 presumably.

Is there a way to tell clover to use a different resolution/ aspect ratio?

I have this same situation.
Already tried change gui resolution using Clover Configurator, but it didn't worked.
What I did was disable clover timeout, so it goes directly to Mac OS.

Try disabling CSM completely in your UEFI BIOS. You might be required to set OS to Windows 8/WHQL. Make sure to disable Secure Boot if you set your OS to Windows 8.