I was careful with the weight rating on my aftermarket wheels to be sure I didn't de-rate my truck. As Shiner says, same goes with tires if you change the size or type.
My take on a half ton with a fifth wheel is you can do it with the right combo but there aren't too many options that will fit the bill in terms of the particular half ton or fifth wheel. That said, beef up the tires and supplement the suspension and you can probably cheat a little on your official payload rating. At the end of the day you'll probably be happier just upgrading to an HD truck, though.