RE: iSonic or Carve as Lightwind Board
I agree with your opinion that the Isonics ('07 in particular) are the most likely boards to suit your "plane early and achieve good speed criteria.
At 78 Kg. you are a relative light middleweight so you may be able to get going on a slightly smaller Isonic than you realize.
I know the '06 Isonic 115 was pretty surprising to me in that it planed somewhat earlier than I anticipated and was plenty quick on the top end.
Your choice here isn't going to be easy.
If you select a larger Isonic (145,155) you will get better early planing, but lose a bit on the top speed.
If you select a smaller Isonic, (133;122; 111; 101) you will get a board that planes as early as an equivalent sized Carve, with significantly better top speed potential.
You will give up a little looseness and versatility over the Carve boards however.
If you aren't interested in "carving" in small waves, and can learn to control the Isonic as higher speeds, I think you will find them bo be just what you are asking for.
The Isonics jibe quite nicely (I was surprised at this as well) but perhaps not quite as easily as the Carves.
So, my recommendation for your weight would be the Isonic 122 or 133 if you have winds in the 10-15 knot range alot and some larger sails, or the Isonic 111 or 101 if you have higher winds.
Hope this helps,