RE: Formula or isonic 155 for max speed ?
Ian offers a good response, but I would ask him to offer a little more.
I have no experience with the 155, but have raced the F175, F147 & currently the F160. Speed down wind in 20+ knots is mostly dependent on sailor skill, especially if the water is rough.
In flat water at 20+ knots, the 155 is likely to be faster, but what about in 1 meter chop? The board that is easier to control, probably translates to the fastest board?
Formula boards are amazingly fast down wind in rough water, but it takes a tremendous amount of skill and confidence to make it happen.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.