I told you it's not an issue, it's by design, but I didn't deny the existence of a difference. Other cars include the Firebird, Turn 14 BRZ..
Ill explain it again, make sure you understand what you read. On these few cars, the customizable transmission stays full manual, meaning people with a clutch pedal can still use it with the custom ratios, and people who don't have a clutch pedal still have to wait for the game simulating the upshift pretty slowly. The vast majority of cars are given a dog clutch transmission when installing the customizable gearbox, meaning the clutch can't be used on it and upshifts will be very quick compared to most stock manual transmissions.
I think it's cool that some cars stay full manual with custom ratios, if I had a clutch pedal I would probably use the M3 all the time, but it would be better if we always had the choice between dogclutch and full manual.