Until (if?) GL_UNSIGNED_BYTE is handled by the hardware, using CPU to convert the data will introduce a slow down. As you see, the more HiRes (so more pressure on GPU, less on CPU), the less speed difference, so CPU used for Data conversion count here.
Also, minigl is, I guess, made for Quake3 in mind and is probably heavily optimize for this engine. I don't it's completly fair to expect gl4es to be faster here (even when GL_UNSIGNED_BYTE handled by hardware).
What gl4es will bring is more function, and faster speed when using advanced OpenGL functions (like TexGen or shaders), I don't think you'll see any speed advange of gl4es will using idTech3 based games, or simple game with low geometry and no complex opengl renderer.
Remember Neverball is much faster with gl4es (and this one use TexGen).
You may see also benefit from gl4es on SeriousEngine or maybe TORCS also (or SpeedDream too). Foobillard++ also should works better on gl4es (if it even works with minigl).