Either way you slice it for that model year it is essentially their 2500LD, yes it's 1500HD shares the majority of the same things but we are arguing over semantics.
Point is coming from that specific model year between the 5.3 with 3.43 gearing and the 6.0 and 3.73 gearing it wont be a "huge" jump in performance. He is gaining a few HP and some more torque, but the gearing will allow him to get into his RPM range faster and hold gears better.
The significant difference will be the suspension, and drive train but that comes at a cost. The truck's curb weight is a bit more than that of the 1500 which almost negates the HP / TQ "performance" gains he will be getting.
Again, you will notice a big difference in the suspension and drive train, but your not moving mountains performance wise. If you don't agree, go actually test drive the difference in the too. Oh I also owned a 2002 Chevy 1500 with the 5.3 back on 2004, so yes I know the power difference on the 5.3 4-speed and the 6.0 6-speed for the NBS as I have owned and towed with both.
Be realistic on your expectations in understanding you are getting a substantially better drive train for towing, working. Your going to have to rev it just like the 5.3, your gaining about 50tq and 30hp subtract that from the added weight and you get the idea, gearing will help.
