I do not think extra simple materials or textures have much impact on UI performance.
However i recently tried material with voronoi noise generated in real time, and such material had negative impact (but it was quite heavy and transparent).
So for simple materials you should worry about texture size. Only complicated shaders will have big impact on ui.
Also look into UIMaterialLAb (FREE example from Epic), it has great nodes and materials for UI. You can make procedural frames, that are pixel accurate, scale perfectly to any size of frame, etc.