RE: iSONIC 101 TOO BIG FOR 6.2M AND 73KG??
I have a similar background as you have, with wave and freestyleboards. I got my iS101 last summer. I agree that with powered up "smaller" sails, the board get a handful. I have not either tried a smaller fin, but I'm sure that would help. However, I think technique is the big thing here. I have only sailed the board 5 or 6 times yet, but I'm allready improving when it comes to controlling the board. The right trim is super important. If you have to "fight" things, the board quickly get a handful. I run my straps at the "2 position from the back. I tried the rearmost position but actually did not go faster (used a GPS to check).
But maybe some more experiences slalom guy can offer some more insight here.