Having pulled the same 18 1/2 TT all over the Rockies at altitude for the past 25 years, with GM trucks ranging from a 76 K10 with factory 400 SB, to my 06 TD, the TBI had very good torque off idle, but much over 3200 RPM's while climbing the pass it wouldn't go any faster. I could turn more RPM's, but it didn't go any faster regardless of what gear I chose.
I own a 01 Sierra 1500 4wd 4L60E/3.73's with the 5.3 (and it currently has 96k on it), and it does not pull the trailer up the pass quite as easily as my Vortec 5.7/4L60E/3.73's K1500s. The 5.7's have a wider power band, and as far as I can tell the torque rating of the 5.3 is a bit optimistic.
When I went from the factory P245's to LT265's on the 01 Sierra, it slowed acceleration down quite a bit. When empty, even with the LT265's, the 5.3 will beat my 5.7's stoplight to stoplight. But when it comes to pulling the pass the 5.7's do better, and the trans doesn't hunt and peck through the gears as much, and they do it with 600-800 lower RPM's.
I also owned a 2017 Silverado 4wd CCSB with the 5.3. Nice truck, drove it for several months and flipped it for a profit. This truck is a LOT heavier than my GMT400 ECSBs. But even that doesn't account for how it didn't go up the pass as well as I expected. I'm a bit skeptical of GM's HP and torque rating of those current 5.3 motors.