I have one big horse with Cushing's and a Welsh pony (12hh) that's positive as well. My big guy is 31 and is not on any meds. We tried Pergolide but he did not do well on it and is fine without it. He does graze on my lawn--not a whole lot of grass--and is mostly fed orchard grass hay and a mix of pellets (timothy, bermuda, and Nutrena Sr.).
My pony is also insulin resistant as well as Cushing's and is on Pergolide. She is 27 and has done very well on it. She is on 1mg (worked up to that dose) and NEVER is allowed to graze on any kind of pasture. (I test my hay and she gets orchard grass hay that is less than 8% NSC. She also has done okay with the WellSolve Low Starch--just a little bit to mix with her supplements.)
For a great source of information please see the Yahoo Cushing's group. There are many, many awesome people in that group that can provide a wealth of information. (Liz, aka Hosscrazy on LB, is a moderator of the group.)