I appreciate the input. I guess it must be about how long it's in each gear (though a tune can change that at-will, no?) and not the delta change in ratio. A .4 from different gears could have different results. Anyone with more knowledge is free to chime in.
I had a '95 and did the 3.08 to 3.73 switch and really liked it. MPG on hwy didn't seem to change at all. On my'12 w/Brembo brakes I decided to order it with the 3.55 as the 6 sp MT only has 6th gear as overdrive, not 5th & 6th as the Shelbys do. It works well for me on the street or road course (Grattan Raceway). I do need to hit 5th gear as the rev limiter kicks in at about 120 mph in 4th. I hit 129 mph on the straights.
The best bang for the buck in car performance is always gears, engine HP is usually headers, but these days the ecm and intake would probably be the cheapest HP/per dollar
3.73's make a big difference in feel from 3.31's. Bear in mind that your speedo will be way off afterwards. The tune has to be tweaked. Not sure what the dealer will charge to do that, but if you already had a handheld tuner and tune it is a simple fix.