Thats real life my friend. In this game its nothing close to what it is IRL.
A stage 4 professionally tuned GT-R in real life would batter pretty much all A1 cars featured in TDU2 on a track. That is a 100%.
Stock it already beats tons and tons of cars higher than its class. (IRL still here).
However in TDU2, a 458 can easily beat it around a normal track. It seems to have a bad handling in game for some reason (or maybe not as good as it should be) and also since I have done a drag race between the two, the only part of it , that it wins , is the beginning due to AWD.
In real life it will eat the 458 in any category, so I dont think the game's choice of where to put the GTR is under/over powered at all.
Now on the Caterham matter, I can't say anything I do not use it. As for the Atom matter, I find that it should be an A2 car.
My veyron SS eats Atoms on a reasonable track easily. I am not talking about a track with 50 corners, but a track that has straight-ish bits and wide corners. Atom has no chance.
I raced my stage 2 atom v8 versus a stage 1 veyron grand sport I think it was, and sorry but no matter how good and precise i took the corners on the straights he flys past me. And I wouldnt call myself a slow driver either as im in the top 80 drivers on Instant challenges and a few tracks im top 100 ish. ( dont know if you would consider that good but since im using a keyboard + manual I find it very pleasing knowing I won over someone with a G25/G27 ;) )