Extremely doubtful. High range can barely go up a mild slope on a paved road, let alone sucking PTO power away on softer turf and on a potential slope. Low range at 4 MPH is not up a slope, it WILL be reduced to 2-3 MPH depending on the slope.
You will add significant time to your mowing experience by trying to leave the fast moving Zero Turn or fast moving Garden Tractor and going to a SCUT or small CUT. There just is no way around it.
Step up to a 40 HP CUT with an 84" mower that can run 6 MPH on level ground in mid range and now your talking about matching the speed of your ZT. Likely not beating by much, if any.
I tried finish mowing with a tractor (72" Land Pride) and gave up that idea and sold it because of the issue that you are eluding to. Speed.
I was previously (and still am) running a 7 MPH Husqvarna Fast Cut garden tractor, I found that my 4 hour bi-weekly mowing would jump to 6-6.5 hours using my tractor and finish mower. I sold the mower and went and bought a Cub Cadet XT3 GSX mower that cuts at 6.5 MPH and I am now happy. All around, the Cub is a better machine for every measurable metric when talking about finish mowing grass compared to a SCUT/CUT. Also, now that I have the Craftsman (Husqvarna) Fast Cut (Hydro Gear G730) and have the Cub Cadet.... my wife and I can now BOTH be out in the pasture at one time and the time to cut is massively reduced. Screw the CUT and mower! 2 fast moving mowers is the kill deal!
Faster.
Less grass damage.
Less soft dirt tire ruts.
Much more maneuverable.
Much more stable on my severely sloped property.