Those FLT tests are always performed on cool days and are only 8-10 minutes in duration. More horsepower can tow faster until the cooling capacity is maxed out, then the nannies kick in and reduce power, or the engine WILL blow up or squeak to a stop. FLT should perform those towing tests on the hottest days of the year in heavy slow moving traffic. I would never tow up hill in drive or tow mode with my foot to the floor just to prove a point. Instead I would put the selector in manual and go up between 3,000 and 4,000 rpm at part throttle. Faster trips up the hill will use more fuel for a given engine and weight combination, so the MPG comparison is flawed too.
Instead of more power, the 4.3L could use the V8 max trailer cooling capacity, an 8 speed 8L90E and 4.1 - 4.56 gears in the diff, and climb the hill @ 45 mph instead of 60 or 70. Try the same trailer and truck package on a hot day up and down a 13% grade and report back how it worked out for you.