No your def not wrong in thinking that nor is any one in this thread wrong just different opinions , just have to really think about it and look at and consider , when are you ever under say 2400 rpm and at full load with a large tune ? Once WOT the shift points dictate the rpm the engine can be exposed to and when not WOT the truck is seeing relatively low power , thats the main point to address de torquing with shift points , it is the only thing that can control the rpm range the engine is allowed to see aside from being on a dyno , once @ WOT even with a stock tcm tune you will never see under 2600 rpm unless you have high gearing or large tires , and even a S475 as a single stays fully lit at that RPM , the only reason the twin "can" make higher torque is that they can light at a lower rpm , once lit I don't think there is any torque difference of note since the air flow numbers are close between the 2 ( S475 is actually more than a stock / S475 by a bit ) , the only exception is if a guy was to roll into the throttle in 5th gear at the right moment and that can be fixed with a quick change in TCM .
The next problem to consider though is that to really be able to take advantage of the desired RPM range to get out of peak torque ( with out loosing " average HP " ) a larger turbo / turbo's are required then usually would be specd :bdh:
Another to consider if a guy really wanted to easily keep out of torque TC lock up could be an easy fix , but would certainly hurt performance until X MPH ( what ever a guy decided the sweet spot was to allow it to lock up )