From what i have read into this (and considered doing it myself) when i upgraded to the kaaz diff is in normal final drive (stock) 6th gear is only for motorway driving and saving fuel, it is not needed/used on track as 5th gear can cover the max speed your gonna do.
(In my HR 7500 rpm in 6th equates to around 210mph)
so what a shorter final drive does is makes the whole gearing shorter, so redline in 2nd gear would be 50mph rather than 60mph and so on. This will improve your acceleration (as long as you have traction) but means more gear changes. It also makes full use of your 6 speed box but means cruising down the motorway might be at 4k revs rather than 3k (depending which final drive you change to)
My issue was and still is:
1- The cost involved (as above, around 2k unless done which other associated work)
2- The general day to day driving is compromised .
3- It may work great at one track but not at another (ie, where you could go flat out 3rd suddenly now means you have to change to 4th for a second then back down.)
Apologies if you know this and you probably do but just stating what I considered. I think to sum it up, like you mentioned, if your after a drag/full track/nurburgring machine its def worth considering. For anything else probably not.