as you know, I'm heavily involved in the sidecar-cross scene and sponsor a team. we only have two 20" tyres to choose from: a Pirelli and a Maxxis. Maxxis have recently made about 1,000 tyres and then destroyed mould. Maxxis have said that. so soon, ScX will have to get by with one tyre. apparently, 1,000 tyres was a mornings work for one machine and barely worth the effort.
the wider the tyre, the less penetration you get. if you were to load the front end with weight, make it handle, make it grip etc with no loss whatsoever (and it is always a compromise) then it might be better. but the steering would be heavier, the grip levels would throw the bars around, the bike would be heavy to steer and the settings compromised because of it.
on the Sc, Rob found that with a 2.15 front rim, the grip level was the same with a 21" and it was better over rough ground than the 20", that 21" absorbed the bumps better due to the diameter. he felt he needed more time for testing. the actual difference was just 10-15mm in height and the steering and suspension were totally altered. it took three sessions to make it handle with a change of front loop position and front shock change to keep the original ride height on the front.
regards
Taffy