APT - It's great you posted this! I found a similar article late last night that was a year or so old on a truck forum talking exactly about this.
As you depict, and what the other post discussed was exactly the point of having transmissions with more gears with lower rear end gears provides similar final drive ratio as one with less speeds and higher rear end gears. Lower rear end gears will come into play on acceleration out of the gate but at cruising speeds, everything should be even and if needed/wanted, individuals with more gears can shift higher if appropriate and even gain some fuel economy.
I was going to circle back around here this morning and ask the question but it seems as though you have a nice spreadsheet there outlining everything - even for non GM trucks. You should make a sticky with your content as I'm sure as manufacturers continue to innovate on the transmission side, rear end gears will become less a discussion topic. Not only this, the content clearly outlines invaluable info for newbies like myself ;)
We are now looking at a TT right at 5000lbs dry - An used Roo 25RS which is about 300lbs lighter than what we were originally looking at. I know it's just 300lbs but it does move the needle further away from max tow capacity. I'm figuring loaded will be around the 5750 mark.
Huge props on the great spreadsheet!