Looking at their dev guidelines page, they don’t have any texture resolution limit other than “don’t use very large textures on very small objects”, so I doubt that the project has any really hard caps.

supertuxkart.net/Texture_Guidelines#texture-detai…

Do not use large textures for small objects—this wastes video RAM.

If the game supports it or could support it, might be possible to have a separate high-resolution-texture package.