It really depends on your setup. There's no question running 1 guy is far faster than 4. But the patch 1.0.7 helps us out quite a bit.

The issue we have is that we have to stop and pickup items, and sometimes switch characters/screens to do it. So no matter what mp you're on, you'll need to do this - So this may take an extra 10-15 mins a run, in addition to syncing up guys etc... Then probably another 10-15 mins to deal with all the loot after a few runs.

So it's much more effective for us to do higher mp levels, since we spend quite a bit of time looting and checking for loot - regardless of mp level. However, in 1.0.6 high mp is crap exp, so it's not more effective from a paragon leveling standpoint.

1.0.7 fixes this quite a bit, so we can do higher mp's which will take more time, but we'll get more loot and exp. Much better use of our time.

Rolling through mp0 just takes too much time to sort through drops - much less likely to get a good one as well - so lots of junk, which wastes our time. Unless you totally ignore drops, and not many people want to ignore drops x4.