Might not be 'that' obvious in the difference if you're running at 1080 resolution.
What should be noticeable as a big improvement will be things like draw distance, extra particle effects, lighting and definitely shadows.
@SharkyUK might have some suggestions of certain game options that will stress the card somewhat?
Whacking up the resolution is definitely the best way to cripple the GPU.
Modern graphics cards are stupendously fast when it comes to pushing polygons. Even the most modest GPU can pump millions and millions of polygons per second at a high frame rate. In fact, polygon counts aren't really so much of an issue these days (certainly not like they were yesteryear). Per-pixel shader/render complexity is the main factor when it comes to performance, i.e. how complex are the calculations going on in the background (on the GPU) that allow that single pixel to be presented to the screen with that particular colour value. Obviously, the higher the resolution the higher the overall complexity and the higher the performance hit on the GPU.
On a related note, ray tracing... Ray tracing is actually easier to understand and implement than the often complex shaders and techniques used in today's largely rasterised games. However, trying to marry ray tracing and rasterising together (as with RTX, DXR, etc.) can be quite painful - which is why I find current GPU tech so exciting and how technology is evolving to work around the prohibitively expensive costs associated with ray tracing. Ray tracing is orders of magnitude more computationally expensive than our current hardware can handle in real time at interactive frame rates - hence the hybrid rendering methods and the reasons we are only seeing a small subset of what ray tracing (or path tracing) can offer (such as reflections or GI).
EDIT: Sorry, kinda off on a tangent there.
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