I kept a log this year for the first time ever, and it was very instructive to look at the results at year end. My results are not as impressive as Ken's except that I sailed a few more days: 74 for me versus 66 for Ken. Anyway, what I learned is probably obvious to everyone but me, namely, that the smaller the board and sail the faster you go. My best speed on my formula board was an unimpressive 22.2 mph, while on my 116 it was 29.4 (still not blazing fast, but I am working on speed).
In my log, I recorded the weather conditions and also whether I would have been happier had I rigged a different board or sail. From that I learned that my optimum light wind combination is my 8.5 meter sail on my 116 liter board. By optimum I mean that combination would have worked almost as well as my formula and 9.5 on really light wind days and much better than the formula and 9.5 when a sudden storm blew through. Plus it was just a lot more fun than being on the big sail/formula combo and didn't hurt my back as much. I know everyone won't agree, but for me (68 kg), it is true.
In any event, I highly recommend a log. It will tell you a lot about your equipment choices.
Thanks, Ken, for starting this thread.