I wrote a reply last nite, but it's disappeared.
But I very much agree with Alex and Ken.
They both make very good points.
You may have hit part of the answer when you questioned the big 52 cm fin.
The iSonics are not formula boards that use huge 64-70 fins in amazing (up to around
30 knots) windspeeds.
They are fast slalom boards, and each of them has an optimum range and at 24+ knots with a 6.5 m2 rig you were definitely well beyond the design range for the iS 133.
Put an 8.5 m2 rig on, in say 12-18 knots, and I think you will find the board is very quick to plane, very fast, and pretty easy to sail fast. They also jibe rather well.
Ken's suggestion that most boards will have upwind tendancies when sailed in far too much wind, with too much fin, and a rig that's well below the design range is right on in my experience as well.
Try the board in it's range, with a min. 7.5 m2 race sail, and I think you will find the upwind issue goes away completely.
But I sympathize with you. When we get a new board/toy, we all want to try it out as soon as we can.
Funny, I had a board shipped to my brother's house on the West Coast USA, and when I got off the airplane from the east coast, all I wanted to do was try that board.
Problem was, it was dark by the time I got to my brothers house and picked up the board.
Ahhhh... I thought, I'll just take the board down to the Marine Stadium and sail under/next to the bridge, which has street lights.
I must confess, it's amazing what we'll do to get out there on that new board.
Little did I realize how quickly the street lights on a bridge fade away as you get further away from the bridge. I will not repeat that trick I can assure you.
Hope this helps,