From the info I read the Parker 280 wheel motors on the Hyper drive turn 266 RPM where the old White unit turned 250 that is why the new mower goes faster. If a mower loses speed part of it could be worn tread on the tires making the wheel no go as far when it turns a RPM but that is only a slight difference. More than likely the pumps are down on pressure not allowing the wheel motors to spin up all the way? I have not researched it so I cannot say.
I am glad to hear this report because when I tried a new Super for me I realized my XR7 unit could do the same and probably more as the new mower if I updated it.
From what I can tell a Generac uses the same whole pattern as the SB Kohler makes more power than a BB Kaw or Vangaurd and they are testing fuel injection from what I am told + they are made in America.
The drive is easily updated on the old mower as well and it is like 150 pounds to 200 pounds lighter and lower so for steep mowing it should work better.
Yes the new mower is better but for me not really.
You like speed you should try the 26's on the rear, that would be one awesome Z.