The overall benefit to render time would largly depend on you render settings and the complexity of the materials.

If for example you're using complex shaders for your skin then covering them with clothes could speed up things. But if the skin texture is simple it won't help much with render times.

Making parts invisible inside clothes won't do anything if you're using P7. But could help in later versions if you have SSS applied because it appears that SSS is calculated for materials even if they are behind clothing.