Just my comment:
Changing the final drive should surely be primarily for track over anything else. It may not add much benefit 0-60 if any with an extra gear change in between but your never going to be launching from stationary more than once on a track (unless of course you hit the gravel
)
Better final drive should be all about maximum torque/power on exiting corners. On a standard box you may be maxing out in 3rd on a bend but not getting optimum pull on exit in 4th which is where this mod is really going to improve your lap times. You could also argue that a correctly set up track car will probably require different ratios/final drive for each track. 5.1 for example may actually be better on some tracks than 5.46. Closer doesn't always necessarily mean better.
I'll hold my hands up now and say I have no real life track experience but in terms of driving simulators (forza & GT5 I know I know
) if you shorten down the final drive too much you'll do slower times and it will be a different FD for each track dependant on the corners mainly. It's just a game but there's definately truth in that!!