Can't please everyone.
All things considered I think it'd have been just as fun/boring. Substitute the fun of a variety of competitors in classes with more classes but completely homogeneous competitors in each class. No, I don't think either one is clearly better than the other. Apples and oranges.
How are any of the GTRs less authentic than they would be if they'd been added on their own? They're all made up cars.
Especially with the game continually WIP.
So race on spec-racing servers. If the new cars are designed correctly, each one will have some appeal, the same way real cars each have their unique character. Corvette, Porsche 911, Elise, prototypical english roadsters like TVRs or Wiesmanns, Caterhams, FWD hot hatches, and so on. Each car would have some appealing characteristics that you couldn't get in other cars. And anytime you're doing more than single round racing (leagues), it starts to even out in the championship points. And there's more than a negligible number of players who simply like a particular car type, regardless of their handicap. You only guarantee that some cars will be garage queens if they're redundant. The monotony of redundancy is exactly what multi car classes avoid.
Single-model races might guarantee close racing (provided you've got skill-matched drivers) but also make for some bland races compared to multi-model gigs.