I had an '03 Suncruiser 33V. I've hauled it up and down some serious mountain passes at altitudes you won't find in the southwest. I never had a problem with overheating nor with the brakes - and I've gone down some steep stuff with it.
As to horsepower - you can always use more.
Unfortunately, there's not a whole lot of headroom in this setup to do it.
First of all, there's the Allison 1000 tranny. It's pretty much limited by how much torque and power it is rated for and the 8.1 Vortec is right there so any extra power may well void and tranny warranty by over stressing it.
Secondly, the engine undoubtedly has potential to crank out more power but it would require more engineering at the factory level to do so seeing as how everything is pretty tightly controlled by electronics nowadays. There are steps you can take to optimize ANY engine. They generally consist of opening up the airflow on the intake and exhaust systems. Banks Engineering makes kits for many engines to improve power and driveability of them. However, the 8.1 in the W series chassis isn't seriously lacking in these areas. You can gain some improvements with these kits but you have to wonder about the cost effectiveness of them. And again, you also have to consider what happens when you exceed the power rating of your tranny.
I solved my problem by going to a DP. That's a major problem solver but it isn't exactly what you'd call an inexpensive fix.