Okay, here's the math.
The percentage change is tire revs/mile will be the same as the percentage change in the effective axle ratio. (Longer legs means less towing power/torque).
BFGoodrich LongTrail tires:
275/55R20 = 652 revs/mile
275/60R20 = 630 revs/mile
652 minus 630 = 22
22 divided by 652 = 3.4%
3.55 axle ratio minus 3.4% = 3.42
So your 3.55 axle will "feel like" a 3.42.
I suspect you wouldn't be able to feel the difference.
Only if you were loaded right up against the GCWR of your pickup, with a trailer that weighs around 9,000 pounds, should it make a feelable difference.
I've got 3.55 gearing on a 2010 FX2, 5.4L with the 9800 GVR package.
No, you don't have a 9,800 GVWR pkg. The most Ford offered was 8,200 GVWR available only with the the HD payload Pkg and 3.73 axle ratio. You probably meant 9,800 tow rating which your truck has if it has the 145" wheelbase.