Also not from the team, but here is my take on the issue. The iS 133 is a VERY big board for an 80kg sailor in 24-28 knots of wind with a 6.5 sail and 52cm fin. I have an iS 111, weigh 79 kg and find that 20-24 knots is the upper limits with a 6.6 Maui Sails TR 4 and a 42cm fin (should be on a smaller fin - 38?).
With any board in overpowered conditions, they tend to head up. If your back hand is overpowered, you naturally sheet out which takes the pressure off the mast foot and causes you to head up. Trying to turn a board downwind in overpowering conditions is hard to do if you are in both straps.
Another issue may be getting into the back footstrap. At least on the 111, with the outboard straps, it's hard for me to get into the back strap without heading up a bit, then once in the strap with weight on the back foot, the tendency is to keep heading up.
I think it will just take some time for you to get used to the board and I am sure you will find it much easier to manage in 12 - 18 knots which is where it will really perform. I doubt that you will have problems in lighter winds, but the iSonics handle differently than free ride boards and can be quite exciting when fully powered, more so than than the freeride boards of similar volume.
The first time I test rode a formula board was in 20-25 knots and had the same problem you describe. I bought one anyway and learned how to sail in in lighter winds.
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.