Why is anybody surprised by this? The PI system in Forza had always been a sliding scale, based on the slowest and fastest cars, stock, in the game. Since there are different cars in this game compared to FM5, tunes will no longer give the same PI, though they do give the same HP/weight stats. It takes a single moment to look at any car that was in FM5, to see that the stock PI ratings are all different.
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I mentioned the vinyl issue in my review. So far, it's isolated to the lowercase "a" in the first typeface for me. Any livery you import with that shape will hang the game at the saving screen, and if you import a layer group with it, the same thing will happen if you back out of the decal design mode. One telltale sign is that the aliasing of curved vinyls won't work anymore, until you select the problem "a" and replace it with the same shape. Then everything works fine.