You can sure put a lot of ballast in those!!
That was what originally attracted me to the 31x15.5-15 size was the sheer weight gain potential. That size is typically found on large trenchers, some large boom lifts, and even piggyback forklifts so they are usually designed for a TON of weight, which means the tire itself is very heavy. The heaviest ones i found were ~95lbs.. for JUST the tire. Then add the 20 gallons of fill they hold and that's 170-215lbs depending on what you use. So for me going from unfilled 29x12.5-15 turfs to the heaviest 31x15.5-15 and heaviest fill, i could have gained over 250lbs PER REAR TIRE. I could have added 500lbs to a 1000lb (bare) tractor just by running those tires.
In the end i opted not to because i really like the ride quality ive enjoyed with my turfs and atv tires on small-diameter 10/15" wheels combo (vs having r1s on 12/16" wheels on a b7100 which rides like crap) and didn't want to give all that away with all the structure it takes to make a 95lb 31" tire.. so i went with slightly larger turfs, same light construction, did add tire fill, and still gained ~300lbs total.
For me the weight gain is mostly about loader counterweighting and dry ground traction without relying entirely on 3pt implements. And my b6100 with FEL and wide turf tires still has more traction on dry ground than the b7100 with r1s but no FEL. Weight is king. Maybe in mud or soft ground the r1s would dig in enough to offer better traction, but around here that's.. not a common thing. Mud is once every couple of months maybe, and soft ground is the day
after the mud. Every time it rains i get in a big hurry to go do dirt work before it dries up and becomes a big PITA again. But im sure not worried about getting stuck in the mud. I think i've only done that once, and the mud was a giant hole in the ground i was digging that filled up with mud after a rain.