I am probably mowing at 12 to 14 mph, extremely flat field area, but there are a few tractor ruts that do cause slow down. The point I am trying to make, is that I bought the rotary cutter to maintain this field, and the zero turn mower is the best choice of equipment for me in its efficiency in mowing.
Anyway, zero turns are the way to go for efficient mowing. Buying used commercial zero turns is often a better choice, than purchasing a new residential rated zero turn. For me, the Kubota ZD326 and ZD331 is my ultimate wish to own some day. But even used, these machines run $8000 to $12,000.
But nearly all zero turns manufactured today are of high quality and reliable equipment, and most any choice of brand will do the job the OP presented in his original post. I think the fastest zero turn is the Scag Cheetah rated at 16mph mowing speed. Now that's incredible. Theoretically, the Cheetah can mow 11.6 acres per hour.