Interesting. How would you characterize each franchise's handling of weight transfer, lift off oversteer in FF's, abs/noabs braking, tire modeling and aero (top speeds included)?
Gt5 problem was that - even after the patch, 2012, there were rolls of suspension, but it was only a visual effect on driving is not affected. Therefore gt5 had big problems with the movement of the weight of the car (with the soft suspension) under braking and cornering, it is an important part of driving was missing in GT5. In F4 suspension was not perfect, but much better, it is possible to cause a skid by using of heavy braking, or even abrupt cessation of pressure on the gas pedal.
FF oversteer in F4 was perhaps too excessive, but I took it with enthusiasm, because it's better than nothing in GT5. Forza motorsport series - characterized by a slight increase in the level of difficulty of driving and oversteer, it was in the F4, F5 in this sozraniv. However, F5 deep drifts began to occur less frequently, but more painful. In F4 is corrected easily, F5 is too long and hard. even if you do it right, the car continues to lose control, even though the front wheels have long had to find grip on asphalt and return rectilinear motion, avoiding the crash.
In F5 (in the sim physics) slides less than F4 (who called driving simulator on ice), F5 little more ring than drifting. But the funny thing is - it's not clear for some reason F5 physics perceived harsh and strict, and "GT 6" - a friendly and soft. But when I compare them with fast switching, it is not so! "GT 6" is a bit more demanding of skills, more dangerous driving, probably because of the suspension. F5 little more simple and easy to drive. In "GT 6" loss of control is sloppy taxiing, causing the suspension and discharge buildup. In F5 loss of control is most often caused by sloppy work the gas pedal.
So you can not say-that skids and loss of control in the F5 occurs earlier and more frequently than in the "GT 6" This is about equal. But as for FF cars - I noticed an interesting thing. In general, similar to oversteer, but in the "GT 6" many FF cars that have something like anti-roll bar. For example Abarth 500, Mito (I went a little front wheel drive cars so far) And in the F4 and F5 - Abarth 500 has a hell of oversteer, it's hard, but fun. And in the "GT 6" this car is very difficult to lose control and slip 180 degrees. When you try to shake the car, it rests on the invisible wall stabilizer bar. I'm not sure - it is realistic or not.
This greatly simplifies the process of driving, but makes it look interesting. In F5 driving I Abarth 500 recalls the bullfight, in "GT 6" as though he had forgotten to turn off the system stabilization. But not all FF cars act as lead in "GT 6", for example, Clio RS 2013 (if I'm not mistaken model) Clio as damn oversteer like all FF cars in F5, and no super powerful anti-roll bar will not save it from the accident but skillful taxiing
I always go without abs, and F5 and "Gt 6" is roughly comparable, but Gt - I use CSR -E pedal, and it's not fair comparison. I am concerned that in the F5 significantly shorter braking distance and braking process simpler than in "GT 6" Aerodynamics - in F4 is not. I tested the Peugeot 908 and F4 no downforce, changing the speed.
In F5 and "GT 6" aerodynamics and downforce - is. The faster Peugeot goes, the better he clings to the road, but reluctantly gives in maneuvering. In "Gt 6" car refuses to obey steering until you reset the speed to 250 kmh. In the F5 is the same, but not so strongly expressed. Also it seemed to me that the Audi R8 V10 with increasing velocity behaves like Peugeot 908- clings to the road, but worse maneuvers
Tire model in "GT 6" and F5 in my at the same level. On the steering wheel difference I felt. It works well, it improves the driving, but the difference between F5 and GT6 not found. In F4 model tires less perfect than the F5 and GT6 and it shows. It's funny - the difference in the behavior of the tire is more noticeable when playing on the joystick. The joystick seems - that GT6, and F5 it works a little differently.
without dead zone in the center)and this game is running on my netbook with 1.7GHZ single-core CPU and 1GB RAM memory.
If you look at my recent builds, I have Camaro Z/28 that was built to replicate Laguna Seca lap record, it even get close to peak speed at braking points and the peak G's reading of the real car ( I included replay file of the test run and data of the speed/G's around the track for reference ) . I still have another car yet to be released with same close accuracy at Laguna Seca, Audi R8 GT replica, build with real life specs and alignment.
I'm very much agree to what you have said, when I read what APEX book said about car physics and setup, the result in GT6 is not the same, ride height, camber, tire physics and weight + aero modelling still need lots of work to resemble what the book said. Even the suspension setup guide at the last chapter of the book were hit and miss if adapted in GT6. I really hope Kaz will fix the physics - ride height, camber, springs + damper, and maybe tire model 
on the next big update, spec II 
Forgot GT6 can't get the stock spring rate value of the real F40 
Damn, Kaz !! 
Why can't we have GT with AC and FM infused together 
