There will be difference in lighting, ...if PD doesn't pull something like loading AO, textures, baked shadows etc for time pass and/or dynamic weather...
If they change lighting, then game shouldn't be much bigger then it already is (talking about GTS with current content, not new GT7), but if they go for additional textures, baked shadows, AO etc, then one can expect a lot bigger game files, and game in general will take much more space on old gen consoles...
Regarding FPS, I don't see need for it to be changed if latter is going to be used, maybe there will be frame drop here and there while loading new assets, ...maybe even without frame drops can this be done, with some minor graphic optimizations for less stress on loading...
If they decide to use different lighting, aka fully dynamic with all calculations, I do expect FPS to drop a lot, probably we'll see game running at 30FPS, which I don't mind TBH...
There is also another question, will GT7 be using FSR, I expect it to use dynamic FSR on both PS4 and PS4Pro, with lowest settings at Quality, aka 720p (1080p FSR) for PS4 and 960p for PS4Pro (1440p FSR, and then upscaled to 4k). Would make sense using this, since avg FPS boost on this settings is approx up to 20-30%, which means, old gen consoles could get 2x more power regarding FPS drop, from 60 to 30, and additional 20-30% for FSR usage (this is scenario on GTS), which would get it up to avg 2.5x render capability (again talking for GTS with 30FPS and FSR on Quality vs current 60FPS and no FSR).
Also there is this, will they keep same amount of cars? 16-24?!? In GTS there was multiclass SP event with Gr4 and Gr3 cars on Maggiore (I think we drove 911 RSR for an hour as a license test), and game was running just fine 30 cars on track, and considering console needs to do the thinking for 29 cars and physics for 30 cars, instead only physics when running MP game (although not sure if AI thinking is that much of additional CPU cost in GTS)... There were few teases for GT7 where multiple class cars are seen in same race, especially outro in newest gamplay/trailer reveal, where Gr3 and Gr4 cars were driven on LeMans (24h of Le Mans maybe)...
If they keep GFX settings same as in GTS for GT7 on PS4 and PS4Pro, with above mentioned FSR and FPS drop, one can hope for good looking game (with unnoticeable shimmering due to the FSR) and possibility to run much more cars on track with dynamic weather and day/night... And as I said, only drawback is FPS on 30, which again, I don't mind for old gen consoles.
And then there is this, most modern games use effects to mirror (trace) objects in mirrors, windows, puddles etc. GTS, well, PD in general is always a bit different, they for example use inverted geometry attached to current geometry in order to render mirrored objects, for example interior cockpit of the car on windshield, track reflections in puddles etc. So apparently it is more cost effective using it this way, so I presume they will degrade those reflective objects in order to squeeze few frames here and there on old gen, where on PS5 they've could use RayTracing for this.
Now regarding cars, in general, frame time, memory available etc is always same problem for most game developers. I myself did a lot of mods for Assetto Corsa, and there was one rule for car mods, folder of a modded car needs to be max 50MB heavy unpacked. AC (original game) in general uses approx 50MB car files, so everything when loaded for single car is 50MB of VRAM memory, and game alone was meant for 22-26 cars per track, and with all settings bumped up, it should use up to 3GB of VRAM, so I did a lot of experimenting, and basically I managed to run N24 with 100 AI cars and all together it used approx 7.5GB of VRAM...
So, I am not sure how much RAM uses a car or a track in GTS/GT7, but I am sure it has similar analogy. Also I don't know what is the amount of RAM that game uses on PS4/PS4Pro, but there is for sure room for more.
And, now that I mentioned AC, in AC every car has multiple predefined LODs, they take up space, and are loaded dynamically, in GTS, there is only one LOD per car, and sort of tessellation algorithm bumps details up or down depending of distance from camera (asked details of car model). Not sure how much CPU/GPU time it takes TBH, but it is for sure more intensive then predefined LODs.
All in all, I think game will run just fine on old gen consoles, and PS5 won't suffer anything regarding GT7 releasing on old gen, this is my opinion...
