A "correct" tune from GT5 works 95-98% in GT6. (GT5 being 100%)
The problem is if you don't tune yourself, you don't have a real basis
to build off of. IE if you don't know how to correct an issue how do you know what the real issue is or what is correct? With that in mind the tune that you thought was good might not be in the first place.
There is no real rating of a tuner or tunes.
Yes there are some Tune Contest. There are some "Popularity" list. Also most don't list when the tune was made. (Version)
At the end GT5's physics were pretty correct.
We might not have everything but the general principals of it's engine and tuning inputs were correct. So if you tuned a "balanced" car in GT5 said tune will carry over.
First thing I noticed with GT6 is tires.
Sidewall chatter is now in effect. Which is great. Then I noticed aero is different. No reason to ponder as I am sure things will change even more over the next 2 months.