Generally speaking, a lower FPS at the same bandwidth should indeed increase the overall image quality. As you're basically spreading that bandwidth across less frames and as a result have more bandwidth per frame.
As far as lowering resolution goes (OBS' Resolution Downscale); up until a certain Downscale it should indeed also improve image quality. However, this is highly dependant on the game. For example, WoW reacts quite poorly to downscaling, its fonts become quite hard to read. In general, things that are tiny or thin, such as fonts become harder to read when downscaling but the gameplay itself should overall still look better. And if you're playing something like Dota 2, well, Dota 2 reacts quite well to downscaling.
As far as your example goes, 1080p@2000 Kbps I would simply never do. 1080p is very, very CPU and bandwidth hungry. If you are limited to 2000 Kbps, I would suggest starting off with 720p@30 FPS for a clear image.
