"THIS one goes to eleven..."
:lol:
The gains of having multiple gears begins to diminish above 5 speeds. That's why you have seen many uncontested claims that 4-speeds are better than 3-speeds (they are), 5-speeds are better than 4-speeds (they mostly are), but then it starts getting muddy.
Is a 6 speed better than having a PROPERLY spaced 5-speed for the application? It depends - how big is your torque band?
On the Honda S2000, the torque band is like from 6800-7800 RPMs, so the decent working range is prolly 6200 (peak torque minus 500-700 RPMs) to redline minus about 200 RPMs. That's a pretty small working range - so to range properly, you prolly need 6 speeds. This motor's peak torque is not huge either, meaning the power comes from engine speed.
On the MX-5, similar ranges, peak torque 5000 peak power 6700 RPMs, working range about 5500-6800. It MAY or MAY NOT be an advantage to have a 6-speed, and is prolly application (read here: track) dependent. On an autocross course, sometimes shifting the gearbox around is not worth the x-tra torque.
Now, if you have more torque, and/or a larger working band to work with, you can take advantage of this and reduce time lost by shifting and eliminate the extra gears.
Base SKY has peak torque ~4800, pk power ~6600, but very strong torque at peak torque. The working range for this motor is prolly around 4000 - 6700 RPMs. Questionable whether adding another speed in the mix will be an advantage, ASSUMING the gears that are there are spaced properly. In 2006, 3rd gear was too far away from 2nd gear, and almost too close to 4th. This is supposed to be changed for 2007 model year for all manual trannys. :thumbs:
Now, the SKY RL has peak torque at ~2500 and produces this torque all the way to 5300 RPMs

. I would expect this still pulls some past the peak power at 5300, so the working range is prolly 2100 (when the boost starts to come up) to maybe 5500. I doubt that adding an extra gear will do much for the RedLine, except giving you something to mess up your track times... :lol:
Of course, there will be someone who begs to differ, and that's fine too. This is just my technical interpretation based on what I've learned over the years.
Another thing to consider is that the extra gear adds weight, too. That's why you don't see many 8-speed manual trannys out there - the added weight and complexity aren't worth any "supposed" gains.
It all boils down to this: the transmission/RER/tire size system's purpose is all to keep the engine operating in the maximum range of torque and power. The "distance" in the rpm curve from the max torque to max power is important to consider.