Pulling power is rarely the limitation with 5th wheels...add in the newer engines putting out crazy power compared to 20-30yrs ago along with the 10speed transmission is likely better able to make use of the power compared to a 10yr old F250 with 4-5 gears to choose from...and yeah it can "pull" that much without significant issue.
At 8000lb loaded, that's a pretty light 5th wheel. Pin weight probably around 1600lb, so as long as you don't go crazy with other gear in the truck it's within load limits..
Nothing surprising about your results.