I stand corrected regarding the Mistral OD not doing much to promote windsurfing world wide. It did and had a pretty long run. Its strength was in the early years when longboards were still big, but faded off considerably even when it was still the Olympic board.
When I raced longboards (last one was the Equipe II XR), I raced open class and rarely used a 7.5 unless it was blowing over 20 knots. My largest sail at time 10.6 Neil Pryde. I stayed pretty competitive with the course / slalom race boards as they evolved over the years, but when the Formula boards came out, the long board was toast, so to keep up with everyone else, I moved to Formula.