Why is it that Unity planes have so many tris? As far as I can see, there is no way to change this, and it sometimes seems unnecassary to have so many tris in a plane. When I just want to import a static image (for a foreground of background), I was wondering if it would make more sense to import a plane from Maya with just two tris instead?
I understand that the extra tris of Unitys planes arn't exactly going to bottleneck my game, but I'd rather not throw them around needlessly.
Any thoughts / tips would be greatly appreciated!
I don't know the answer to the question "why". My assumption is that the built-in plane has many vertices because a heavily faceted plane gives more usable results with vertex lighting. However, I already answered you here that yes, you should "model" your own quad, and not use Unity's planes.
When you use GameObject->Create Other to create a plane (or a cube/sphere/capsule/cylinder), the editor is creating a GameObject for you with its built-in plane mesh attached (from unity default resources). You aren't able to change the subdivisions because this mesh is not really special in any way (aside from being built-in), and appears as an arbitrary mesh to Unity, just like any other one you might import from Maya.
As you guessed, if you want a 2 tri plane, you should import your own from Maya.