I have the 2008 isonic 111, so I can't speak about the 2009 model. However, my favoriate sails for the 111 are 7.6 and 6.6. I have used my 8.4 occasionally, but for me, it seems a bit big for the board. It works ok, speed is fine, but it isn't real comfortable for me. I weigh 79 kg and usually sail in light to moderate chop in area lakes. I use Maui Sails TR 3's and 4's.
I don't slalom race much, so if I need the 8.4 to get the 111 moving, I will probably be on my formula board instead. Our winds are highly variable when in the 10-15 knot range, so the formula board will be on plane much more than the 111 in these conditions.
If you have steady winds where you sail, you may get more use out of an 8.4.
Hope this helps.