I have a ym2000 I also own a 4 and 5 ft Bush hog. In very thick stuff you can overload a 5 footer in 6th gear and have to go to 5th. With the 4 footer you can run 7th gear all day in all but the thickest stuff and then you can almost run 6th gear in very thick stuff with the 4ft. I leave the 5 connected now and if it gets thick I just only take a 4ft bite.
Choosing a bush hog for a ym2000 involves more than how big of hog the tractor will run. The engine might be able to run a five or six footer but the physical size and weight of a five or six overloads the tractor. Even with a five footer you will need weight to keep the front end down in certain situations. A four footer is a much better compromise and when the day is done, you will have cut just about as much.
I decided to go with the 4 footer, it seems like all around, the smartest, best, as well as, the most practical decision for me at this point. thanks for all of the input guys.