RE: q on isonic and sails
I am the same weight like you, and I have a Sonic 85 (2006) that I use with 5.9 and 5.5 race sails (first with MauiSails TR-1s, now with TR-3s) and a Tectonics Goldwing 28 cm. I agree with some of the posts above: at 26-27 knots, you should still feel very comfortable with a 5.6. So definitely use a smaller fin before buying a smaller board. If you like your Goldwing, go for a 28 cm, it works beautifully with the board. I think that you may not need a smaller sail once you use a 28 cm.
However in case this was not sufficient: I disagree with some of the replies concerning the gap; a 0.5 sqm gap is definitely not too small between 5.6 and 5.1, you will feel a very significant difference. While it is true that you would be better off using a smaller board than the iSonic 87 with a 5.1 m sail, I think that it is still OK (but I have not used that combo before, because I always use smaller boards with a 5m sail).