are you talking about a raspberry pi 2 or 3? i’m going to assume you mean a 3, although i played mario kart 64 at 1080p on the 2 in 4 player, although it is fairly choppy on some tracks, and has a lot of sound distortion.
it sounds like yours is already playable at 1080p? i didn’t say it was 100% full speed! right now i’m running it with gles2n64 at 1080p and it is pretty choppy in 4 player but playable, if you don’t mind the sound distortion. in 2 player it only has very minor sound distortion and small amounts of hitches. it seems fine to me.
interestingly, i tried lower resolutions in gles2n64 and it didn’t seem to make any difference. i’m not sure what causes the choppiness there but it’s evidently not the fillrate or CPU (i checked CPU usage throughout and it was pretty low)