This isn't in any way a relevant answer to my post, but I'll answer yours anyway.
1- Overall driving feel I agree GT6 is the best (but still crap comparing to other better sims). However, even GT1 properly simulated aero drag at high speeds, which GT6 doesn't. Also GT1 simulated turbo lag, which GT6 doesn't. Some tuning idiosyncrasies in GT6 have been present for almost the whole of GT series lifetimes (reversed ride height, wrong camber, simplified LSD), and PD isn't showing any signs of wanting to rectify them. Rubbish.
2- No mate, not by a long shot. Try the Normal Car Cup in GT1 and come back to me. If anything, AI in GT series have progressively gotten easier.
3- If you mean the star positioning, yes I agree. Doesn't really contribute much to a sim though and just shows you how out of touch PD's priorities are.
4- Personally I find GT5 image rendering to be crisper and have better draw distances, but GT6 engine is more "foolproof" for the future and we should see the full capabilities in GTS. But anyhow, GT has always been great in graphics, I've never disputed that. Again, doesn't necessarily make it a better sim.
5- Not really. Even with performance restrictions it's still wayyy too easy to overpower the AI. The only events that are hard in GT6 are Ayrton Senna and Sierra TT, and they are both time trials, not racing against AI. GT1 had more aggressive AI, GT3 had the fiendishly difficult (and long) Formula GT Championship, GT5 also had a similarly difficult FGT Championship, Vettel Challenges and Grand Tour Eiger/Toscana race. What does GT6 have? Even the Vettel Challenges in GT6 is a joke compared to GT5.
You can wait for GTS, but I have played the demo in GT Academy event last year, and besides graphics there is really not much change from GT6. Physics are 95% the same, FFB is even worse, AI are still numbskulls and sounds have improved ever so slightly. Oh, and there's even less content cars/tracks wise and career mode is literally a hyped up license test. Good luck competing against Forza 7, PCARS 2, AC 2, etc