Umm...correct me if I'm wrong buttuh...

I haven't used neither GI Joe or MR but isn't GI Joe used to fake GI (global illumination)? MR can do the real GI so why would you want to fake it? Ok, granted there's most likely some render time issues but still...

The grainy look you mentioned is imho an unwanted side effect of bad quality. I would want to get my renderings as noise free as I can. You can always add grain in post if you like...

Just my $0.02...