I would only agree to participate in such an event if the cars were the same ex. mx5 cup car, etc. With GT3 the cars are very different one to the next 'out of the box.'
If we look at the times of the last races, we had a pretty good mixture of cars in the Top 5, Top 10.
Like Audi, 650s, GTR, Z4, Lambo and Ferarri.
You can look up the exact standings in sgp but I just combined the last 4 races I have been in.
In the Top 10 we had:
McLaren 650S - 11
Audi R8 - 7
Nissan GT-R - 5
BMW Z4 - 5
Ferrari 488 - 5
Lambo - 3
Porsche 911 - 3
SCG 003C - 1
Road America Top 3
Bahrain Top 3
Okayama Top 3
Kayalami Top 3
So I would say we have a good split of the cars. Sure the 650s is currently a little more popular, but the figures show that every car (except the AMG) is capable of hitting the Top 10. Also in the Top 3 we already had 5 different cars.
They all have their advantages and disadvantages but time wise they are all pretty close. I would also argue that if you pick a different car and you get some practice in, you will hit similar times with each car. You might just prefer one over the other and stick with it. Thats why you might be faster with your favorite one.
The time difference is just personal skill level. Just look at @Chris Down , he wins with the 650s, Ferrari or even the SCG.
Watching our last races, I would say there are currently 4 groups of drivers battling.
Our Alien like Top 3 followed by a group that battles for places 3 - 10 and then I would say the back markers are also divided into two groups. So there are enough people to race with a very strong midfield currently.