So... About the GT7 Tire Model
It was too much to hope for. The tire model in GT7 is wildly different from GT Sport, the last game we ran a multi-tire series in. In some quick testing I did at Michelin Raceway Road Atlanta, I found there to be nearly 2 seconds per lap difference between each of the hard, medium, and soft tires (so about 4-5 seconds between soft and hard). The difference in GT Sport was more like 1 second per lap, fairly consistently across all tracks. The difference in GT7 is so large that I'd expect the races to devolve into three different races, one each for each compound, and the three groups won't be anywhere near each other on track.