I hadn't been using any camber as of yet as I have felt the lack of grip it gives to some cars.
Have you not experience the grip fall off when running even mild camber ?
I used to run similar amounts of camber to you on my drift cars in gt5, but I'm not having the same result in 6.
Camber and the amount of roll your car has are directly linked. The stiffer your front end is, the less camber you can run. I like to run a soft setup usually (again in both GT6 and real life) because you can get more traction and be more aggressive with the car. However, as you stiffen, you have to run less camber. For instance, a lot of competition drift cars run no front anti-roll bar, this allows more roll, meaning you can make use of much great camber. So in GT6, if I have to run the car too stiff to use good camber, I soften the anti-roll bar on the front

Every tuning parameter has an impact on another, so you can't just say 'run 4 degrees camber on any setup' because you will have to tune springs and arbs to match camber on any given car. It takes a lot of practice to get a car perfectly dialled and this is another reason I love GT6, the realism is insane. Unfortunately, because people are struggling, their ego's would rather let their brain think it is a problem with the game and not a problem with their inputs.
GT6 will just constantly screw the handling during drifting with random grip changes and such things.
I've not found this at any point, I found it a pain to start with, but the simple fact is that it is a perfect match for real life except for one thing.
In GT6 you have two sets of feedback. 1) your eye and ears 2) the feedback through the wheel
In real life you have three sets of feedback. 1) your eyes and ears 2) the feedback through the wheel 3) g force.
Being able to drift in real life first means people learn to rely on the g-force aspect, once that is taken away, everything feels alien and wrong. I did it the other way, learned in games and then went to real life, so I do far more with the two which we use 'in-game'. Basically, if you can learn to drift in GT6, drifting in real life is EASY. The inputs are all EXACTLY the same as real life, except you have added feeling from the g-force, meaning that suddenly you have a huge amount more awareness of what the car is doing.
I drive a GTS Skyline in real life - it has a viscious LSD and no rear sway bar and yet I can drift it all night long - however in GT6 as soon as weight transfers fully onto one side the tyres immediately give in and the car spins.. if my car did this in real life I'd demand the previous owner refund me my money!
See above

It's because you can 'feel' the car and chassis behavior in real life, everything else is the same
I am seriously contemplating trying to run a GT Academy style competition for drifting in Europe through my drift academy. To try to take some of the best drifters on GT6 and see if I can find one to make into a drifter in real life........ It would be awesome to see if I'm right or not. I just can't work out how to make a quantifiable and fair qualifying format.