Interesting, I hadn't heard about that. In the case of Mario 64 though, many of the models and polygons didn't have textures but used super-simple single-color

Interesting, I hadn't heard about that. In the case of Mario 64 though, many of the models and polygons didn't have textures but used super-simple single-color flat shading. So texture fill rate wouldn't not have been a constraint. Most games also didn't use a custom microcode, but used one of a handful of microcode libraries for the RCP that Nintendo provided. A notable exception to this is Factor 5, which did write their custom microcode to improve graphics performance, with great results.