It will be less if you have a high kH, since the bicarbonate will act as a buffer and prevent the pH from dropping too much.
No, the "buffer" isn't really a buffer. It doesn't resist pH changes. All it does is shift the pH that the water assumes with CO2 in it higher. The change in pH is the same per ppm of CO2 whatever the kH is. i.e. the ppm of CO2 needed to give you a certain pH is directly proportional to the KH of the water.