Hi Gas20,
When you say "add 2 tires" and "extended fenders" you're really describing a conversion from single rear wheel to dually, not just 2500 to 3500.
Dodge in prior years on the single rear wheel models has almost no difference between 2500 and 3500 as you can tell if you compare payload specs. This is in line with your observation that all they did was add 1 leaf to the spring pack. But payload increases dramatically when you go dually 3500. They are no doubt beefing up other components to get that higher spec.
There was a post recently that Dodge has changed this in the new models shipping soon.
Not to start a truck religious war, but if you buy a GM or Ford 3500 SRW, you'll get the payload spec you were expecting without having to buy a dually. Or you can wait for the new Dodge and see if they really changed.