Yes, you are a hardcore hardcore fan lol. You don't have any SC character!
From my humble experiences, I'm sure it was far off more efficent to farm in MP1 with magic find or exp gain items for "legendary farm" or "experience farm" than with MP10 with highest DPS and EHP equipment. I say this for old system:
With patch 1.0.7 exp gain changed considerably like this:
MP 1: 25% (up from 10%)
MP 2: 50% (up from 20%)
MP 3: 80% (up from 30%)
MP 4: 120%(up from 45%)
MP 5: 165% (up from 60%)
MP 6: 215% (up from 75%)
MP 7: 275% (up from 95%)
MP 8: 340% (up from 115%)
MP 9: 420% (up from 135%
MP 10: 510% (up from 160%)
As you see even after patch 1.0.7, highest MP levels aren't viable for experience farm because at MP10 you gain 5.1 fold exp bonus for 34.39 fold monster health (bonus for monsters lol). It's hard to keep DPS growing with monster health. (34 fold at MP10!) I think picture is not different for Magic find also, because Magic find grows slower than exp bonus with MP levels. In my opinion, sweet spots linger between MP1-MP4 even for "godly" players. After MP4 you slow down considerably. But that sweet spot search test would be worth inspecting. I wish I had time to test it (I farmed most of the time at MP1 with my SC Barb) and felt it was most efficient.
But with new multiplayer gains, cooperative farming sweet spot may be several MP levels higher than single player one for a 4 player farming situation. This needs to be tested throughly also.