Take this as speculation, but educated speculation none the less.
We can all assume the PS4 will be more powerful than the PS3. Otherwise they'd just be making a cheaper PS3. We can also assume that a (say) 50% price increase will yeild a considerable performance gain. Again, we can also assume that with the Vita's specs and trends in hardware (alright parts becoming dirt cheap), there should be at least 1GB of ram in the thing, MAYBE 2. As for the Processor, sony probably conducts some kind f pegan ritual (only explaination).
Now, for development, let's say we want the cars in-game to be GT5 Photomode quality (meshes and shaders). Well, for 200 cars, that's done. The shaders are already fantastic, the premium models are the highest quality seen in a game as of now.
For the other cars, the shaders won't have to be re-done. They all (standard and premium) use the same materials, otherwise they wouldn't be cross-compatible. So every paint and finish is basically taken care of already.
As for modeling the cars to a standard, there's this wunderful thing called the smooth mesh button in Maya (software they use last time I checked). For the majority of the car, once they get the form right, they just mash that thing until they get roughly the polyount needed. Base mesh is 100k and you need 400k tris? Mash that button, and you get a silky-smooth car model. Sure there's some tidying up (actually, a fair bit of tidying up, with antennas, light jewelery, keyfobs, etc). But you get the picture. The longest part when modeling the car is the start, once it looks good at GT5 quality, it gets exponentially faster to make it look good at better quality. And since they'll be using GT5 as a base to start from, then that's 200 car models, done.
Similar thing with the tracks. Frankly there's nothing wrong with the mesh on the whole. Maybe some white lines on the outside need smoothing up a bit, easy. Perhaps they want prettier water at Monaco. 100% chance they already have it, it just didn't make it into GT5 because of performance issues.
From here on, it's adding to an already fantastic base. Maybe they want better tyre marks on the tracks, more spectators, rougher sand traps. Shadows, well, that's just a setting they'll tweak. More cascades, higher resolution. That's literally two numbers someone will change, test and then put away (until the final six weeks looking for another 2fps).
Will they go for SSAO? Probably not. Fancy Depth of Field like the new NFS? It'd be nice, but not nice enough for the cost.
I think they're at the point where, with GT5 looking fantastic when it's at it's best (Spa, Nurburgring, etc), everything they do will look like a better GT5. They've made the quantum leap to >200k poly cars, stunning real-time lighting and awesome detail.
You're not going to see a GT4-GT5 leap in visual quality again.
but yes, I want them to become 
