With that preface, you can see each car's "PP" value, which is the same concept as the "PI" value from the "Forza Motorsport" series: it is a theoretical number averaging each car's track time potential across a variety of courses. According to GT7, the GR Supra is way out in front with a PP value of 544.49, followed by my current car at 508.53, with the GR86 taking up the rear with 489.07.
To test the three cars, I did six successive laps around Tsukuba with each of them via the game's time trial mode. The results were like this, with an asterisk I will get to in a second:
The results fell roughly in line with the game's calculated PP values: the GR Supra was way out in front, my car was in the middle, and the GR86 was very close behind. But now is where subjectivity comes into play.
I actually gave myself seven laps with the GR Supra: the car was such a handful on a tight, twisty track like Tsukuba that I was having a really difficult time being consistent. After six laps, my best time was 1:05.356-- but the segment times were all over the place: on one lap I would nail one section but have huge tire spin on a key corner; on the next lap I would would bang off the limiter exiting a turn due to how crazy fast the engine revs; then on another lap, I would do well but carry too much speed into the final curve and go off into the dirt.
The GR Supra is a pretty heavy car by Japanese standards, and even with the sophisticated suspension and the nice way it rotated, it was very fussy. It felt like there were so many roads to failure that putting together a nice clean lap was a chore. I did take the car around Laguna Seca as well, and on a higher speed track with less demand for corner carving the car felt much more at home.
Next let me discuss the 350z. The car is about the same weight as the GR Supra, but has nowhere near the engineering sophistication: it was almost impossible to aim the car at an apex and actually hit it. The car felt pretty flat but understeered a ton, and just didn't want to change direction very well. I was able to be more consistent with it than I was the GR Supra, but it sort of felt like I was wrestling with a bear the entire time. I still had fun with it, but Tsukuba gauntlet really exposed all of this car's weaknesses in a huge way.
I've been driving this car in real life for the past thirteen years. It felt pretty convincing in the game overall. The car does feel heavy, with weighty steering, and does tend to understeer until you whip the back end into line with the throttle. It's more of a blunt weapon than a precision instrument.
Now let's discuss the GR86. From the first lap to the sixth, I was able to get extremely consistent times, getting just a bit quicker each time as my skill increased. You point your eyeballs at an apex and the car just goes there, with zero drama or delay. The car feels much more agile and precise than the other way, and I far and away had the least frustrating time driving it. While I wasn't able to match the 350z's best lap time, my average lap times were vastly more clustered together.
In general, the GR86 was by far the most enjoyable car to drive on Tsukuba. But on a higher speed track it would definitely fall behind the other two cars, especially the GR Supra which would leave it in the dust.
I also fiddled around with GT7's "scapes" photo mode a bit, to get a sense of the three cars' proportions compared to one another.