True COG relative to the back bumper is the same. However, COG relative to the front wheels is 18 inches off. From the perspective of putting the same weight on the rear tires, it's probably close. But wherever the COG is, it's 18 inches farther back from the front wheels which will be less stable if you consider the front and rear combined.
In effect, you are forcing all stability to come from 18 inches back. Before you say it works fine, remember that there are people here who have 2000lbs over GVWR and C rated tires, and it works just fine. :)
Seriously, given the choice of keeping the 2500 with 19.5s or buying a long bed DRW, I'd keep the SRW and do the upgrade to 19.5s.
