The only time I've so far seen repeated penalties for what appears to be only 2 wheels over the line(and still well over) is at Algarve as @
prisonermonkeys suggested and at Sepang South in the tight hairpin, second to last corner. I'm still not sure I don't get 4 wheels off by a tiny bit, but it does seem for sure touchier in at least those two corners. Everywhere else I've been sure(after getting over the anger) that I was legitimately off and gained advantage. And of course those corners are offset by a couple others I've gotten away with what I think is surely an advantage.
The Sepang one is the most annoying because the AI cars hog the inside line and force you to either get a penalty or get into a shoving match with them. It's a bit like the left-hander after Druid's at Brands, except on the inside and with a penalty instead of a spin -- they just don't care if you're there in a couple corners.
Also, I believe that F1 rule(which is still irrelevant as far as GRID is concerned) is no longer the case unless they've changed it again, 4 wheels over the line is supposed to be a penalty. They used to let them get away with murder and go all the way over the curbs with all 4 wheels on that green stuff. Made certain corners at places like Catalunya just plain embarrassing. As I was told, that rule was supposedly changed to require keeping a wheel or two inside the white lines(which were always officially designating the actual edges of the track) a year or two ago and they have been calling it(and driving it) tighter lately, though they probably get exceptions in a couple places.
We don't get the benefit of pre-race drivers' meetings to clarify the rules for each track... which half the drivers in any given series ignore anyway. We just have to find out where the limits are when we exceed them, then adjust our lines accordingly to remain legal. The game does a pretty damn good job of not giving you a very harsh penalty if you don't cut very badly(except at Algarve).
It does appear that Grid-land is a place where beating up on other cars is acceptable however, but of course is at your own risk. If the alternative is getting penalties for the AI bashing me around, then I can live with it. Just as I can live with the rule [which I agree with] that if you put 4 wheels on the curbing or grass and gain time or position as a result you can expect a penalty.