Here is a supplemental thought after getting out yesterday on my iS 111.
Big sail on a short board = heading up in sub-planing conditions.
I was on a Maui Sails TR4 7.6. This sail on the short 234cm board causes me to put a fair amount of pressure on my back foot whenever I sheet in because of the relatively long boom, which extends well beyond the tail of the board. If I am slogging waiting for a puff of wind and sheet in too soon without heading off the wind, the board rounds up very quickly.
In sub planing conditions, even if I am on a beam reach, the board always wants to head up. I can't slogg comfortably while hooked in waiting for the wind without continually forcing the nose of the board back to a beam reach (front foot pressure and moving the sail forward). Once on plane, no problem heading up, the board goes where I want it to go.
I don't know if this observation applies to your situation, but for what it's worth.......