I have an A4
Upgrade from 11.3" to 12.3" rotor

While you are, in fact, increasing the arm of the swept area, there will be a difference in stopping distance. However, because you are utilizing the original caliper, it will do nothing for pedal feel and response.

I was happy when I did the A8 rotor upgrade. But am content when I coupled it w/ boxster calipers.

$300 for better braking? I think so...