The whole 'better suited for PS4' thing is questionable as to whether it was a legitimate statemetn trying to excuse the lack of premium cars by intoning their ineherent quality or whether it was kind of a tongue in cheek brag. There is no PS4, no one knows what it will be like and such a statement is fairly useless for any real value really. It is the norm to develop a model to a super high detail and scale it back until it fits your processing ability... this is how it's done with many games.
As for how long each car takes, the whole "1/18th model" on the bottom of one ofthe skylines makes me doubt whether they really went through as much as we think they did with their creation.
Although i would agree with you i would just like to point out that it's not something i have said and i'm merely passing on hearsay and that i also didn't use the term 'PS4' but rather 'next gen', it's a small distinction i know and maybe i'm being picky but at least 'next gen' doesn't preclude the notion that the 'PS4' is not the automatic destination of the next GT incarnation be it a 'prologue' variant or GT 'mobile' variant .