I've been driving manual transmissions for 40 years. My wife and I currently have a 2016 Mazda3 with SkyActiv technology. They put a 6mt in the car...and it's ridiculous. For one thing, a 6-speed gearbox is only necessary for a supercar with top speeds over 170-180, and secondly, the gearing isn't designed properly. 3rd is too close to 2nd, and mostly useless. The final drive ratio in 6th is fine, but what would be most useful is to eliminate 3rd, and move 4th and 5th somewhere lower than the points where they are now.
The only reason for the current popularity in 6mt gearboxes is purely a marketing thing.
After driving the car for a year, I've learned that most of the time, accelerating to a point between 45 and 55 at a comfortable, fuel-efficient rate goes like this: 1-2-4-6. If I'm accelerating to highway speeds more quickly, then: 1-2-3-6. Every time I shift like this I find it annoying!
If you're driving in a 30-35 zone without your foot in it, just basically idling along, I wouldn't recommend anything higher than 5th. You're not going to hurt the engine without any engine load, and you're not going to be driving far enough to start carboning issues. At that speed, the difference in fuel economy between 4th and 5th would be negligible. In a 30mph zone I'm in 4th, and 35mph is generally where I move into 5th. I realize this is meaningless without RPMs, but you get the idea. There isn't that much difference in gearing ratios from the Mazda3 and the Z.
The Mazda3 is designed for fuel economy, and I've learned how to tweak the accelerator to squeeze out better mileage than the 5-door version is rated for, by watching the mpg computer. (I don't rely on the computer for final tallies, however.) Even though the Z is a completely different animal, the same principles apply.
My two cents...
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