We have one of those Allis pull behind combines.. the pickup is a whole 40" wide! We've never done clover or alfalfa, but we have done sainfoin, and we got about 400 lbs off a 1/3 of an acre! I just seeded 3 acres in pure Algonquin alfalfa so that if the opportunity arises, I can get seed from it there. I'd expect FAR more seed off it, or it wouldn't pay to do it at all, I would figure if you had a good stand of alfalfa and it set good seed, you should get a couple hundred pounds per acre... you certainly should get more than your sowing rate!
Around here we're so hot and dry the management for seed harvesting is a little different than most other places, I found I get better alfalfa seed set if I cut a first cut early, then it comes up more uniformly. The past couple years we've been direct combining the Sainfoin, this year we're going to swath it... see how that works out for us.