The 6 sp auto has a 4.1 first gear, effective with TC around 7-1 vs the old 4 sp at 3.1 for the 4l60 and 2.48 for the 4l80, effective 5-1 for the 4.60 and 4-1 for the 4.80. right there, alone, assuming a 3.42 ratio. you would need a 5.32 axel gear to equal the overall low with the 4l80, and 4.8 with the 4l60. The usuable ratios are better, along with the splits being closer..........you can probably pull a LOT more than what the gcw rating implies and still be fine!
That truck 'should" do better. BUT like a lot of things. it all depends.
Marty