I took a trip a couple of months ago and logged 33.8 mpg on a 1050 mile trip. Just took the same trip and logged 35.8 on my 2020 10 speed with a Cobb 93 octane tune. I tend to flog it around town, but just drove normally on this trip.
The 10 speed is certainly more fuel efficient in the 2018 plus models than the 6 speed in the 205 to 2017 models.
Those extra 4 overdrive ratios certainly make a noticable difference when cruising.
I also have a Cobb 93 tune, and found that it does not affect fuel economy unless I'm heavy on the go pedal.