I know a lot of people will disagree with me, but I wonder if you would be better looking at the Futura instead of the iSonic!!!
Yep the iSonics will work with performance freeride sails and freerace sails, and I see lots of people using these type of sails on iSonics and they are having a great time, but they are not really going faster than good freerace boards.
To be 100% truthful, iSonics are really designed to be sailed with full race sails in over-powered conditions. This something that freeride/freerace sails don't do as well as full race sails. In over powered conditions, iSonic become alive, fast and fun.
In most 6m or 7m freerider or freerace conditions it would be correct to take an 8.5/9m sail on an iSonic. Not every ones idea of fun, but thats how iSonic like it.
And I agree with Jean-Marc, a 7.2m on the iS137 is not good. 8.5m is really the smallest you should go.
Just my 2 cents worth