When Dodge put the 20" rims and tires on the 1500 they upped the gear ratio to 3.92 to compensate for it so it didn't affect tow rating.
Now as Radio says it's ultimately the tire diameter difference that's going to affect your rating. If the tire diameter reduces in tandem with the rim diameter then IMHO you'll gain about 1000 to 1500 lbs of towing capacity.
According to the Dodge towing site they increase towing capacity by 1000 lbs for approx every .4 increase in axle ratio.
This is just my opinion and in no way is to be taken as fact. I also didn't take into account any other info such as frame stength, springs, axle, etc.
Your Dodge dealer should be able to reprogram your ECM to correct your speedo once you've gotten the tires on it.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!