At my last track day, I used one and a half tanks (averaging a little less than 6 mpg ). Yeah, another pitstop due to a smaller tank would have been annoying.

 

So at 39.2l/100km you would come in 22.98 kms earlier with the small tank...
Saying it another way, with a 64L tank at that consumption rate you would be running it empty every 163kms as opposed to 140kms (approx).

I'd say for convinience sake a 90L tank would be the only one to truly make a difference.

Reminds me a bit about the individuals stuck on gear ratios when they have been about the same in the entire 991/981 lineup and weren't a huge issue before.

 

Almost a no brainer to get the extended fuel tank. Almost no one looks for a car without one. Weight is minor.

 

I used essentially a quarter tank for each 20 minute session, meaning I could run four sessions (if I pushed it) before having to refuel. The smaller tank would require refueling after a maximum of three sessions, plus I would have to refuel in order to get home without fear of running dry. With my larger tank, I can get safely home and find my gas station of choice within the next few days.

Regardless, the car is also used for road trips in fairly rural areas and 10 liters is the difference between fuel anxiety and arriving in the next podunk town with gas to spare. This comes from experience: I've owned a '93 Miata for well over twenty years. It came with a tank around 4 liters smaller than the later 1.8 cars and that extra gallon makes a difference. Fellow travelers with the ever-so-slightly-larger tank would jokingly call me Jeremy Clarkson (a reference to the Top Gear episode where he did a road trip in his Ford GT, which had an incredibly small tank compared to the supercars driven by his co-presenters).