Not sure if I understand the questions the last two posters posted, but the reason for running a machine at capacity is the way the separators are set up.  A good share of the threshing occurs with the friction of crop against crop.  When you start running an almost empty separator you lose that ability.  STS machines tend to throw corn out the back even in best scenarios, (sorry but that is even what techs say) and my experience in corn is a minimum of 1 bu. per acre primarily through the rotor no matter how you set it.  That is running a 12 row cornhead in 250 bu corn on a 9660.  Very loaded.  I'll bet the loss per acre will go up to at least 2-3 bu. per acre through the rotor and as you speed up your cornhead more to accommodate the combine you will realize more head shell.  I have never tried running w/o combine full so I can't verify what I am telling you, but that would tend to be the consensus of combine gurus.  Edit:  I notice 430 deere posted at about the same time I did.  Sounds like he can make it work, so I guess that is the answer and I respect that.