I normally seem to sail in 8-15mph winds on a fresh water lake. The wind doesnt usually get much higher. I have been out in windy and gust conditions >20mph but got blown all over and still couldnt get planing. Roger you were right about the old board dimension (typo) I was estimating the dimension from memory but saw some new boards at the weekend and the old one I'm using is much longer and thinner for the same volume. Also i cant hire a sail larger than 5m at the lake.
I have had one go on a friends board which was a Mistral around 10 years old and approx 145ltrs also. he had an 8m sail which was extremely difficult to uphaul and manoeuver. However I'm not sure on the weight of his mast, boom etc.
I ride with the daggerboard down when I'm heading upwind as I just dont make progress otherwise. I have it up when trying to plane, but as mentioned I cannot acheive this at the moment.
I hope this gives you a litle more information on what might be best for me.