edit: look at bottom of that post first
That 1,2G refers to lower or higher speed? Is that base value for any class? I was able to reach 1,2 only for “higher speed”. It doesn’t feel right to use it as base value for each class.
Anyway, I was testing 2005 Impreza STI in B class (for my own purposes, how good is 2.4 swap and if I can make EVO 9 as fast). so decided to use that data and extend it a bit. Proably not the best tracks for testing, but it is what it is. Shirakawa and Festival I don’t know well, Bandai a bit better, but I’ve used rewind to set what could be “a clean lap”, so take it with grain of salt 
I hope that picture below is self explanatory, but feel free to ask.
Last one is with aero, rest with no aero.
Method I’ve used to reach 1,2G = pick a tire close to 1,2 → get best power/weight without droping to much below 1,2 → adjust aero to reach over 1,2. In the end it was max front and 1/4 rear. Was that correct?
It felt good to drive, but weight + drag was noticeable.
And here is a comparision between 1,2 version and tune that gave me best average times:
Keep in mind, that IMO all of that tracks favours power, so e.g. for any other Touge track I expect that lightest + rally tires tune would win.
EDIT:
Or maybe I’m just stupid and you were referring to Gs on that screen? Yeah, that would make much more sense than Gs from tuning screen. I guess I need to go back to drawing board, oopsie 
