There is no difference (when empty) in acceleration unless you're WOT; what you're noticing is the speed (time in seconds) it takes for each truck to up shift from first, since the 3.92 will reach higher rpms first and upshift sooner, it will feel like the truck is going faster/easier. Each truck has gobs of power available. If you feel the 3.21 isn't speeding up fast enough just push the pedal down harder, problem solved.
The 3.92 is not needed unless you run bigger tires and/or have the payload available to pull something > 8000 pounds.
And when I'm pulling my trailer with my 3.21, first gear is never a struggle. I have never once thought "I need more power" when towing in first. My RPMs are always very low, 2500 max. Third and fourth, that's where the weight begins to bite as I'm usually trying to accelerate on an on ramp or pass etc and they can jump up to 3500 - 4000. But first gear is literally the easiest of all gears for the truck to be in, I see no point in trying to make it even easier, that's not going to make me want to pull a bigger trailer. And quite frankly, feeling 7000 pounds behind my 5th gen I'm not sure it's even safe to go beyond 8000 regardless of what the specs say, that rear end can get squirrely at times.
