Light wind sail size for iSonic 137

Hi!
I have just bought a 2012 carbon iSonic 137 for light wind leisure sailing and I need some help on choosing the right size of sail to match it.

I’m not into racing, but I want to go really fast and do long distances including hi upwind courses (as hi as a slalom board can point – I didn’t want a formula).

I weight about 73 Kg and during the last years I’ve been using a 2009 JP freestyle Wave 107 board matched with NeilPryde’s 2009 Firefly 5,7 and Hellcat 7,2.
With the Hellcat 7,2 I’m able to sail really well (consistently into the footstraps and planning) with wind of about 16 Knts (30 Km/h) and above.

I also got a 2010 Starboard Go 155 to teach my kids and friends, which I also intended to use my self for lighter winds (lighter than 16 Knts), but it doesn’t seem to do much difference: about 15 Knts (28 Km/h) is the threshold to have really good time (consistently into the footstraps and planning) (with the Hellcat 7,2).
Recently I once also tried the Go 155 with a NeilPryde V8 8,5 (I think of 2009 make), but it was rigged with an X3 mast (only 35% carbon), it felt quite heavy and didn’t seem to do much difference in terms of early planning (at least not the difference I expected or rather wanted).
I suppose it also has to do with the shape and mainly the weight of the Go 155: almost 12 Kg.

So now I got the 2012 carbon iSonic 137 and I’m thinking of buying the 2013 NeilPryde H2 racing to match it (8 batten-2 cam “freerace” sail).
Basically I want to use the iSonic 137 for winds of about 15 Knts and less. (for stronger winds I’ll be using my freestyle Wave 107).
I don’t know if the iSonic 137 will work well with my existing Hellcat 7,2 (no-cam “freerace” sail) at say 15-13 Knots of wind.
I will only buy one new sail (almost decided for the H2 racing) and I can’t decide which size: 8,7 or 9,3?
The objective is to constantly plane from into the footstraps with the lowest possible wind but without living any gaps (wind range for which I won’t have a good combination of board-sail with all the gear I will have).

Thanks a lot to anybody who will bother to help,
Neo
Cyprus