It's more the straight line stopping power that I worry about with a big, heavy car.
I imagine most people don't push the lateral grip of their Q5 very hard, but those unexpected heavy braking moments are the ones that concern me.
Having said that, I had Avon's on my old TT and couldn't tell any difference from the Pirelli's that were specced from new. I do wonder how much tyre technology is just scaremongering.