I can't answer the hitch extension question, but when I was researching before I bought my current Dodge, I did learn that the 20" tires can reduce hauling capacity.
This was both on the official Dodge site and on truck enthusiast sites. Since I happen to like Dodge trucks best but wanted to make sure it was the best choice for my farm needs, I looked at every nook and cranny/ins and outs of the Dodges and happen to discover that info quite by accident. I would have never known that on my own.
Anyway, when I bought my truck I had them switch out the 20" super flashy "upgrade" tires back to the standard tires- for which all the specs are written for anyway. Sometimes I haul some hefty loads for my class of truck and needed all the oomph I could get. I also searched long and hard for the sport model hemi because it is made with the better gear ratio for what I needed. My friend has a Dodge similar to mine, but with the larger tires and a different gear ratio. Those little differences give mine the ability to out haul and out perform her truck quite a bit. (Not that I ever rub it in her face or anything.....
It also sits lower and holds trailers in a better position.
In addition to finding out the differences hauling a goose neck makes over a BP, it might be worth while to see if whatever upgrades many trucks have on them to look flashy haven't reduced your hauling and workability of the truck. Good thing to know before trying to haul something with your truck thinking it will work like the specs are written.