I think you just have to understand the cars. The NSX is an outright fast car. You can't say it a straightaway only car because it has the best rotation of any car in the series. And that ability to rotate the car is what makes it fast in the corners. But, its not good in all types of corners. It has a huge weakness on slicker tracks like La Sarthe or tracks with a lot of elevation change like Trial Mountain. It however, owns on flat and fast tracks.
The GTR, is no doubt down on straightway speed, but it is so easy to drive fast its just ridiculous. GTR's do well on all types of tracks with the exception of the fast flat ones (for obvious reasons). The GTR's are the only premium cars that can match a standard Takada Dome, and it only gained 30 kilos. However, the loss to the GTR's engine mod is a big deal. That car responds really well to the engine work.
The Lexus is very well rounded, its given a liberal amount of hp but, I'm not sure why it was given weight. In an open spec the SC is just slower all around, I just don't understand the reason for the weight. I think the turbo and the horsepower figure is pretty good though. I noticed that I could drive these cars pretty hard and they consistently lapped fast. Not quite as easy as the GTR, but the straight line advantage might give the SC and edge over the GTR on most tracks.
The Supra is a great car in this spec. You get the best of both worlds in that car, stable and fast on the straights. In open spec, the car is... ehh... not so good. I also found the car to be really good on tires (likely due to the light weight).
The Nismo Z is a car stuck in the middle and is searching for an identity. It wants to be a high speed straight car with the monster torque but, the extra weight and low hp output makes sure its only average in that department. And as far as handling goes, its a little twitchy and is not as quick to rotate as the NSX but, not as easy to drive hard as the GTR and SC430's.
For the record, I will happily take any car for the final two races if there's a question on whether the car is too dominant.