some big studios have reported a speed increase of about 300%

to get anywhere near the same quality in the native renderer would probably take a lot longer, but it all depends if you are using a lot of renderman shaders

good thing about that paintfx workaround with MR is that the lighting rig is the same, so the end result shouldn't look to much out of place, you might have to change the shadow settings coz MR has it's own shadow section in the light's attributes