I ran a 72" BEFCO finish mower for 5 years behind my
B2910 (very similar to your
B2920 in PTO horsepower, size, and weight). The tractor handled the weight and size of the deck without trouble (either with the FEL on/off). But, in the Spring or when the grass was lush, I had to run in Low most of the time to keep ground speed down, otherwise it was too easy to load up the engine/hydro. We have a good bit of slope here, and a lot of dense pasture grasses. Later in the season when things were dry, I could run in Med mostly.
Just my $.02, but I got the sense that 72" was about the max my 22 PTO HP could handle consistently without putting excessive pressure on drive-train components. I'm not an engineer, but after all those hours in the seat, feeling and listening to the feedback from the tractor in lush and dry conditions, flat and slope, 72" just felt like it was right on the edge of being too much. Sounds like your topography may be slightly different.
I always felt like I wanted to be mowing faster than that 72"/22 PTO-HP setup would allow for my terrain.
If you do decide to go with 84", I'm sure staying extra diligent on blade sharpening and keeping a clean deck will help with ground speed.
Dean