After some digging*, I finally tracked this down to the kernel. Any kernel after 4.5.7 will not provide any resolution higher than 1920x1080 (at least with my setup).
The purpose of this post is to find out if my problem is hardware specific or more general. If you have an ultrawide monitor (or any res greater than 1920x1080):
- Did it work 'out of the box'?
- What hardware, specifically GPU (is this an intel problem or AMD/nvidia as well) and monitor?
- What version of mint & what kernel?
* Without the help of gm10 my knowledge wouldn't be where it is today.