here's a cartest comparison that I ran between 2 supercharged cars, the only difference being the 3.62 diff.
they are almost identical (the 3.62 is just slightly quicker) in all runs that span multiple gears.
if you run some tests without shifts you can clearly see the advantage of the 3.62 gears comparing gear for gear.
in 60-90 the 3.62 car is over 1/2 second faster in 4th gear, but the stock car can actually do this in 3rd while the 3.62 car needs to shift, and in 3rd the stock car is faster than the 3.62 car in 4th.
Overall i think this mod probably makes the car feel quite a bit faster but the actual performance is about the same plus you lose quite a bit of top speed.