I did a test last week. I didn't use 6th gear at all to see what my gas mileage was like. I do about 80% freeway driving at 75 mph, and normally average 30-32 mpg. With one tank full not using 6th I got 28 mpg.
On the other hand I just did a long trip going 85 in 6th for hundreds of miles, and average about 34 mpg, and I'm guessing that if I did the same thing in 5th, I'd be getting around 26 mpg.
I'm also thinking that I could get oversized wheels like 215/17/50's to off-set the lower 6th, but then 1st gear would just be that much higher, and when I put my smaller racing tires on that might make for the higher 1st gear.
I guess my biggest problem with the c-one gears is that they moved 1st gear higher instead of lower. Of course then they'd have to move the 2nd down that much more.