I've actually found the car details/accuracy to be better than Forza with each generation. Like the R32 Skyline GT-R (and numerous others - like most Audis with their front grills) has NEVER looked right in any Forza game (headlights especially) and i consider myself an expert on that view, since i've driven/ridden in no less than four of them in real life, and see them on almost a daily basis - and the headlights on our cars are pretty much identical.
Forza 6 and Horizon 3 improved the amount of detail in the cars over GT6 but the accuracy is still off with some cars though - i just don't get why, they've had so many opportunities to fix it. Whereas i honestly cannot fault any car i've seen in any GT game. Even the accuracy of the cars in GT2 still amazes me, when every other car game at the time (and for years after) had way less polygons, and often in the wrong places.
Then i look at GT Sport photos and just when i think they can't get more details than Forza or premium GT6 cars, they do just that. Seriously - compare Forza Horizon 3 cars, GT Sport cars and the real life versions and tell me which game has the better accuracy. I'm really not being a fanboy here, but it just seems like GT has always set the benchmark for car details and finally now other developers are catching up to them - they've essentially helped the industry try harder.
I think the reason that GT games have so many framerate drops recently is because they put so much detail into the cars that their game engine can't cope (maybe they need to start building a new engine?). Because as we all know frame rates drop when too much is happening on screen.
And remember GT has had changeable time of day and weather in its past few games on certain tracks, while looking better than a number of other games that do the same thing, or less (that i know of). Yet again - not fanboyism, these are just the things that my attention to detail eye picks up on - i'm a sucker for accuracy, so everytime something isn't quite right it catches my eye.
But PD do need to keep hiring more staff, or maybe change/redo their engine like i suggested above - the sounds are still not as good as Forza's even with a Forza sound man there. I'm increasingly happy with the physics in GT, but they still have to work on the low speed stuff. But the way the body momentum quickly snaps the car into oversteer is more realistic than Forza, as i've done the exact same thing in real life on a few occasions - it happens way faster than Forzas slow mo version...
Kaz's problem i think is perfectionism, and it's either the consoles (as per my post above) and or the engine that cannot cope with that. They even admit they have to tone some things down to make the game run better.
P.S. I've been playing Forza games (Horizon 1-3 and Motorsport 3-6) almost exclusively for months now, so clearly i'm not anti-Forza.