IDo the B+20's have adjustable rear tire settings
The answer is no. I have a 2620 (great tractor) I have looked into this w/ my dealer Emerichs and they are sold at the widest possible tractor setting. I have my rears filled w/ rim guard and I have gained about 130 lbs. per tire as a result. The 2920 tires are the same size as the 2620 so you would gain the same. I agree w/ jloaf deffintily reccomended.
As far as your tractor purchase unless you need the little extra PTO horse. you would get w/ the 2920 over the 2620 than I would say go w/ the 2620, lots of power for its class for sure. I have no complaints. Plus the extra 1000 bucks can buy a nice tractor toy or 2. Good Luck!