not sure what the upgraded gear is but basicly the larger number gear you go with ( 3.55 3.73 3.90 4.10) the faster you get out of the hole, but the sooner your rpms top out. yes you can still go up to the speed limiter. and yes it will effect gas milage. but its not that bad. on the down side from a roll, your out of your powerband quicker.
the standard is 3.73 manual and 4.10 auto. but there are some extremists out there.