More ponies out of my 5.3L

Hey guys im new to the site just signed up for it today. I got a 2003 Silverado Ext. cab Z71. Already have Gibson shorty headers, Banks intake kit, and true dual exhaust with Thrush mufflers on it. Im lookin to make some serious power out of it since im considering turning it into a desert prerunner and they usually have a lot of hp under the hood. Thats just a thought right now, nothin really planned out. Now ive been lookin at a top end combo for the truck from Trick Flow Specialties. It claims it can make up to 455 hp but im not sure how true that is. Heres the part number on Summitracing TFS-K305-455-425. I wanna know if this will be a good option or if there are some other better setups to go with. Also im not really wanting to sacrifice gas mileage too much. A little is fine but not a huge drop. Any help would be appreciated. Thanks guys