Reminds me of that commercial with the Toyota pulling a space shuttle. Yeh, it can do it, but you'll need to pack a lunch if you're going across town.
That 5.3 is really a wimpy motor, and pathetic compared to the 5.7. But, you can do a couple of things to help the situation.
First of all, the motor needs to breath. The truck comes with a partial cold air injection. The air box is good as is, but the plenum takes the air from the box to the intake by way of Bangladesh. Replace that plenum with an air tube from Airaid. This more direct air flow really helps out.
Next, ditch the stock exhaust. If you don't want loud, that's not a problem. Gibson makes a nice cat back system that doesn't rattle the windows, and it opens up that much needed air flow.
Those two things alone will help with the horse's and give a slight bump to the mpg's.
Finally dump the stock computer program. There are quite a few chipset companies out there that will sell you what you need for your truck. Personally, I used a Diablo Intune, most the folks over at the Silverado forums preferred this one, and I'm happy with it.
I'm a data junkie, and I tracked the changes on my truck during the process of modifications. Each piece showed improvements in the performance. From an aesthetic viewpoint, I went from a truck that lost 15 to 20 mph on every east Arkansas hill, to maintaining speed without having to pour fuel in by the gallons. And towing MPG's went from dismal 8-9 up to an average of 10.8.
It'll never be a good ole 350, and you are pushing that truck hard by pulling 6500, but it can be made more comfortable.