If I wanted to go fast in a truck with the 545RFE or the 6 speed, the first thing I'd do, long before any type of forced induction or nitrous, is gears.
I've recently gotten 4.10s put in my truck, with the 545RFE, and I am convinced that was the ratio that transmission was meant to have, before the EPA and the Bean Counters stepped in and ruined things with the 3.92 and that worse than worthless 3 55. In first gear, as tall as it is, at 2000 rpm the truck is going 15 mph. My first gear is now 81% of what first gear in the 8 speed is. Not too shabby. At 65 mph with the 545/4.10, I'm spinning the engine 1800 rpm. Perfect. I used to HATE that 545 until the 4.10s. It's still no 8 Speed, but it's not bad...
If I really wanted to go fast with a 545RFE truck, 4.56s is what I do first. 4.10s are a good compromise between performance and economy, bit I'll bet that 4.56 would romp. Matt at Moe's did 4.56s and I'm waiting to hear all about it.
Any transmission is breakable. I have a co worker who busted an Allison behind a DuraMax. But it stands to reason, that theoretically at least, if you reduce the load on that 545 by gearing down, rather than powering up, the 545 would last longer.