My new 757 JD ZTR will mow at nearly 10 mph. I did not believe it would but it does. Now granted the terrain has to be smooth, but units capable of going faster will generally be more efficient than slower ones, even when not mowning at top speed. There are several times already when I have touched 7,8 mph while mowing then slowed down. The fact the speed is there does mean something. To me, the Scag or any commercial based unit within reason is the way to go. They generally have better designed decks and higher blade tip speeds so they will mow faster on varied terrain, even with a smaller deck, because they are made for efficiency. I mowed my neighbor's five acre lawn yesterday for him (he was out of town) in two hours, going around trees and dealing with some hills I do not care to mention. He returned today and asked me how long it took me to do such a "great job". He was shocked at my reply as with two small Honda riders and a Massey 252 with a 7' RFM attached it takes him (and his family) four plus hours.
John M