It's like how forza 2 had it. All you would see is S7's.
But the thing is, PP is better in that regard than the current system. HP and weight
encourages game breaking cars because it ignores 75% of a car's performance.
I've been to a few low hp races online, ~300 hp class. Every so often you get a guy who shows up in a completely legal Rally car or GT300. If the host couldn't kick, it would be a disaster. Also, even harder to regulate are tunes like aero, suspension, and transmission. GT5 sees a stock car in equal light compared to a race suspension, adjustable transmission, racing fly wheel, front+rear wing car. We're lucky (though, more unlucky) that GT does not accurately capture cars like the Viper ACR or Saleen S7 because they would always win in their power class. Those two cars were popular in Forza yeah, but they weren't the only options.
Also note that GT5 will (hopefully) let the host pick PP, so you won't get people aiming for 800PP, because everything from 1-1000 would be used online.
I would love it if the PP system works along with horsepower, weight, and tire restrictions so we can setup races that are unique, to be able to exclude certain cars with the settings.
Yes. Allowing tires to mix was terrible in GT5P. At the very least, GT5 should have an option to lock tires to a specific choice (and no one should have to go and buy all the stupid tires for online either).
Hopefully PP will also allow the cars with odd stats, like the Elise, to more freely enter online. To have a good race in the current system, you need to limit HP and weight. But this often means that Elise like cars can't compete because they are either too light or not powerful enough. PP should let them fit in more easily.