I too am an advanced driving instructor. I am nothing special, but I know some highly respected race drivers and driving trainers that taught me all I know and they all advocated a quater to three grip. This is the grip which most rims are adjusted to fit, with sockets for the thumbs and sometimes the heels of the thumbs too, and with the signals easily reachable by the fingertips. Also, in 8 to-4, the force of gravity is going to pull the arms down in a way that will tire out the arms and reduce the balance of the grip, relative to quarter to three or ten to two.
This position (quarter to three) also gives the biggest possible leverage on the steering column, so the grip is more balanced and the tactile sensitivity is maximized. In terms of emergency response, the maximum amplitude that both hands can reach is 260 degrees of steering, versus about 180 degrees at 8 to-4 or ten to-two respectivelly. In 8 to-4 the amplitude is further reduced because the trunk poses a physical block to the free movement of the arms.
This grip is the most suitable base for most advanced steering techniques I know, including the classic rally driving technique which we advocate in our driving courses, as well as other techniques I have seen in the big racing and driving schools in Europe. It is also the grip adopted by any race driver I have seen or heard of.