Ed, I also talked to the same guy with the 2 50HP Yanmars. He said they go as fast as the XXW4000, but just has more power and uses lots less fuel. I also talked to another guy that has one of these machines. You should come to the Dixie Chopper Chat Room I have on my site and talk to other owners. Terry from Ky. has a XXW4000, the twin 20 Kohler set up and he mows at full speed on big lawns. He mows several large commercial lawns and he also has a 60" Flatlander which mows at 13 MPH. He says the XXW4000 is faster than the Flatlander. From what I've heard, the XXW5000 is as fast as the XXW4000 with power to spare.
We also have another member that has 2 of the XXW2600 models, the 72" with a 26 HP Yanmar and he is very satisfied with them. He mows sports fields with them and is from Maine. You are welcome to come join in our chat room anytime Ed.