Now this point isn't to take away from the fact that the drivers have done a good job, but there's something about the Toyota & Mazda Gr.3's that is beyond BoP to fix, but gives both a distinct advantage. This advantage is why the Supra specifically has become by far the most popular car in the league racing circles I'm associated in.
I'm sure that this has been known by some for a long time, but it's to do with the cars power bands. Take a look at the blue line specifically in each image (apologies for quality).
Mercedes AMG GT3
Peugeot RCZ Gr.3
Chevrolet Corvette Gr.3
Note how the blue line reaches a peak generally towards the end of the rev range, then falls off. This is a common thing for the majority of Group 3 cars.
Toyota Supra Gr.3
Mazda RX-Vision Gr.3
Note how for these cars there's a relatively flat section on the power curve. This means that they are at a higher power through significantly more of the rev range. Not only does this have an effect on acceleration, but it can be used especially in the Supra's case to short-shift to save fuel without losing any time. That's why it's beyond BoP to fix. Whether Polyphony did this deliberately as a sneaky way to give their "official partners" a bit of a leg-up I don't know, but it is a bit suspicious. The Mazda might have something to do with how a rotary engine makes its power, I'm no expert on those. But in the Toyota's case it's almost like it runs on diesel (torque aside).
Peugeot 908 HDi (a diesel LMP)
I hope that this can be fixed in GT7, because it's an absolute nightmare to work around in leagues. It's not like either are actual real-life racecars, just concepts, so Polyphony can change them if they wanted.