Hi,
we had a same issue here and we all made a same mistake. For couple of first time you should not add too much batten tension. Instead of that leave them so loose that there are few small wrinkles around lower four battens and apply downhaul as printed in sail or maybe little less.
After sail has stretched out slightly you may start adding more batten tension and apply 1-2 cm extra downhaul.
That helped me and the other guys here. So now my 8.4 is working just like I expected.
I also have Vapor 11 and 12m2 formula sails from 2009 and they worked out of the box.
I've compared this sail with some older race sails with GPS and I don't observe any huge speed difference against them, but range of this 8.4 is quite amazing. So old 6.7 can barely handle only little more wind that new 8.4, but 8.4 keeps going with good power thru lulls and that's what makes difference in slalom racing. After initial disappointment (because of camber rotation) I'm actually now so happy that I'm considering to buy a smaller 2010 Vapor too.
May a wind be with you
-T
