I have a drake 70cm with the 158. Normally a couple of pumps works but last week I was using the 8.5 which was probably a little to small.
What I do notice is that other sailors don't pump at all - just hook in and ease onto a plane whereas with the 158 I always have to pump. It almost seems that you must use a larger sail with 158 to get her going early.
I do fin pump when I am moving and I am actually get to be a decent 'pumper' now.