Just a thought or two about your situation. Have you thought about selling the ST115 and replacing it with the iSonic111, especially since the volume variance isn't that great between the two boards, yet the iSonic has a greater sail range capacity? Of course, a crucial point of certain is your weight, as it just might complicate things when using a larger sail.
Another thought would be selling the ST115 and picking up the Futura 122. The Futura would cover your desired sail range (5.75-8.5) and offer you a bit more floatation than the iSonic 111.