Just as well the Ka driver wasn't in an ST, sounds like he would have wiped the floor with you.
Not a question of wiping the floor, racing is out of the question on public roads. It was a bit of fun as there was a yawning gap between the cars performance wise. I certainly would not try to challenge a BMW M3 which would be just about the equivalent difference in performance of my ST over the KA.
Had the crazy fool been in a car of similar power, there would be no fun. I would have moved over and let him go. I once saw a guy doing similar things in his 'wipe the floor with anyone' EVO VIII. Eventually his car wiped the floor with the lamp post.