The 6th gear isnt a taller gear as you get with many 5 to 6 gear upgrades. Many people who autoX and otherwise actually swap their 6 speed for a 5 speed.
I've found the opposite to be true, rarely have I see a 6 speed transmission where you have an "overdrive" 5th as well as an overdrive 6th. Unless it's something with an absurdly high top speed and a hell of a lot of horsepower. i.e. Lambos and the like.
Nearly all the BMW M series have 5th as the 1:1 with everything before it being closer together, rather than just adding potential higher top speed. Same with the higher powered subarus. Their 5th is close enough to 1:1 as makes no difference.
For a vehicle that tops out at around 115-120 or so, having a closer ratio transmission, while there's the possibility for a little more rowing, it gives you a lot more time in the sweet spot in your torque band and you don't have an absurd drop from 1st to 2nd.
Regarding autocross the less time you have to shift, the better off you are. I don't think that many of the M3W crew are doing autocross though.
I think for the Miata itself though skipping gears can also be attributed to what rear end you're running. What's the ratio yours is running?
I'm definitely with richard on this though. I love a close ratio 6 speed for overall driving, than a 5 speed with a lot wider spacing.
