I dont know about that. From my experience, if you are cutting an overgrown field, a bushhog will do it in about half the time. You have to go painfully slow with a finish mower to get a halfway decent looking cut from my experience..
LD1. I thought your handle looked familiar
Since he didn't indicate how wide a cut his bushog was compared to his finish mower let's not assume they are the same. Single head bushogs are normally sized narrower than the tractor back wheels while finish mowers are normally wider than the back wheels. If he has a 6 foot bushog and an 8 foot finish mower using the same overlap waste per pass he will make a lot less passes on a 40 acre field with the wider mower.
We don't know what kind of power he is pulling them with so another unknown
to deal with as far as going slow. Have you ever cut with your finish mower at a 6-8 inch height? I grant you if you cut a high thick hay field with a finish mower at 3-4" you will have to go slow and will still get some clumping.
At 6+ inches for the first pass, unless it is rich, thick grass, he will be cutting a lot of seed tops and stems versus the thicker stuff close to the ground.
If it is good hay he is chopping up a lot of money in 40 acres. They claim hay was going from $400-600 a ton in some of the drought states last year. Somebody should buy it from him if it is any good and do the baling, perhaps after it is prime, but still better than sagebrush. A lot of hay in Ohio is baled after it is prime, as you well know. A lot of weeds are baled as hay and sold as well.
Another reason for going high the first time around is he isn't familiar with the field. How many junk car parts, high stumps, groundhog holes, and dead bodies are lurking in the high grass waiting to do harm to his equipment or cause him to be hurt?
Just my opinion, as usual.
Ron