I'm thinking "much" better graphics might not be all that true, of course it depends. Some PC graphics fiends can get all warm inside over a change of framerate from 110 to 120fps, or an increase in resolution beyond 1080p.
I suspect that what NEO will bring to the table will be smaller improvements, like almost no pop in rather than very little, and more light sources in night scenes. As mentioned above, four player split screen rather than two. I don't anticipate more cars on track or dynamic time of day or weather. My inclination on that is if PD can get it working on PS4K, it will work about as well on PS4.
I seriously think GT Sport on PS4 is going to spoil us with having proper shadows alone. If NEO will be able to push graphics beyond 1080p, that would be great, but I'm plenty happy with 1080. If we get more dynamic light sources at night, or slightly better looking rain, I doubt that's really going to be a game changer for anyone, and I'm betting Photo Mode images will look identical, or very close. There won't be a NEO version of Sport, any more than there are different versions of PC games for the new graphics cards. So I wouldn't feel the need to sell your PS4s in order to have a "true" gaming experience.