I test drove an 8 speed PW and they definitely feel snappier off the line, but just couldn't pass up the deal on a '16 model they had on the lot. This was several years ago when the '16 was 3 years old with 24k miles on it. Yes, the 8 speed felt better but didn't feel $20,000 better (yes, I am cheap also...).
The 8 speed has a much lower 1st gear and is basically like driving an older 6 speed PW that has 6.00 axle gears! 2nd gear in the 8 speed is about the same as 1st gear on the 6 speed, and is why the 8 speed "handles" bigger tires on initial take off much better. However highway driving should be pretty much the same as the top 3 gears in either are basically identical ratios. This means that the truck will downshift just the same when hitting a grade or towing a trailer down the road regardless of being a 6 or 8 speed.
6 speed 4,5, 6 gear = 1.00, 0.82, 0.63
8 speed 6,7, 8 gear = 1.00, 0.84, 0.67