Our cars have more than enough torque to blow the tires away with the stock 3.55's. You go much lower than a 3.90 and 1st gear will be all but useless.
I see your point about moving from dealership to dealership because of your service commitment. What branch? I'm retired AF myself.
I'm trying to think of an instance where a service department would come back and say, "Sorry, were not going to warranty XXXX because you aren't running the stock 3.55 gear ratio".
I guess if you took the car in for a leaking or broken differential cover, broken half-shaft, or even a broken driveshaft, the dealer could deny warranty coverage due to the lower gear ratio. Then again, how would they know the gears weren't the stock 3.55's?
U.M.