Touring cars produce less amount of downforce compared to an open wheeler when both cars travel at the same speed. I've never seen a one seater rolling over by hitting a curb and of course drivers do mistakes in real races too. There are plenty examlpes of race drivers loosing control while turning...
And if I'm not mistaken the grass isn't lower than the curb in turn 1. If there was a height difference and the wheel stepped from +20 cm to 0 cm in a split second I would expect a major accident. But in my case in the first corner I don't even have contact with the grass... And even if the curb is 20 cm high it is not an instant step from 0 to 20, but the curb is an angled area.
edit: Look at a BMW rollover crash test --> http://www.youtube.com/watch?v=Z0I4fmttN7Y&NR=1 at 2:00". Look the height of the ramp they use. In turn 1 I had almost the same results with lets say the double speed from this crash test but with a car with much better downforce and a curb which can't be compared with this video's ramp. And as I said if the curb gained 20 cm vertically it would definitely be a disaster to ride it. But the curb is angled. The only thing it happens is that the right wheels aren't on the same level with the left ones. I don't see why this should cause a rollover.