I really like the race tyres and the ability to have different compounds of them. It adds a nice level of strategy to races with pitstops.
Just now I was able to gain several places in TC spec Suzuka by running a hard/soft strat compared to the standard med/med strat.
I don’t know how I feel about the PI value of them, it does seem a bit too low for how much influence it gives.
That it works great on slower cars is very logical. These cars don’t have the power to accelerate, so anything that helps them keep up their speed is of great value to them! The same reason why the MX-5 is such a good car, it’s not a fast car but it sure knows how to keep up the speed that it has.
It only seems the PI calculation doesn’t take into account how big of a deal this is, but as @PJTierney said these calculations can be very complex.
What I would like to know is how the balance falls in longer races, where you can clearly notice the lower tyre wear of non-race tires. Does the lack of pit stops make up for the slower pace?
A great example of this was an earlier Spec Series, the Vintage Le Mans Sportscar Series. I made it a topic (here). The more prototype style cars had higher tyre wear, but were also significantly faster through the corners and slower on the straights compared to the more road cars. Still one of my favourite series that multiplayer has seen!
I really hope Forza adds longer races, even for these lower classes, to see how it plays out when tyre wear and strategies come into play with this!