I think a far more useful restriction would be to prevent race cars from entering races that they don't intend race cars to enter.
Agreed.
It would be a sad sad day if we could no longer put sports tires on race cars, as many of them are much more enjoyable to drive that way, at least with mediums or softs. It would also make an even race with the AI virtually impossible since simply reducing power makes for a ridiculous game of blocking on the straights and parking in the corners.
The TTs just need to have more filters for the leaderboards so you could try to be the fastest in a particular car, or the fastest in an unmodified car, or the fastest in a street car, or whatever. As it is you have 3 options. Pick what you wish to drive and see how fast you can go with it on your own, basically like practice mode -- OR -- Attempt to organize rules of competition between those on your friends list -- OR -- Use the fast car and try to see how you can do on the actual leaderboard.
It's not actually exploiting anything other than the same "fast-for-its-PP" thing that we're stuck with in most online racing anyway, it's just that if they set the rules up that way, then they - like any sanctioning body - are setting the manner of the competition. If the rules favor a particular car, then the competition for overall victory will nearly always be between drivers of that car.
PD knows what the options we have available are, and they know how the past events have gone. So what I guess what we have to accept is that PD actually wants the majority of the TTs to be a test of who is the fastest in a properly detuned 2J / R390 GT1 Road Car / Elise 111R or GSX/R. If they wished, it wouldn't likely be terribly difficult to change that dynamic. As a rule, I choose to find other things to do after getting my easy million and perhaps setting a decent lap in my car of choice.
It would of course be nice if they would ban or restrict the use of ballast and power limiter to encourage use of different cars, or if they allowed/disallowed specific cars per event, but there will almost always be found a fastest eligible car no matter what they do.
Wow, that grew to be longer than planned.