Lots of good feedback. I think the answer also depends on what kind of riding you want to do under 15 kts....trying to do freestyle, wave riding, or "free race" riding, or something else...all somewhat different.
I am about the same weight as you and have pretty good light wind technique. I can sort of stay upwind with my 5/8 fish + 15m kite in 12 kts....with a light wind board (e.g. my Spleene 29 Session) I can get upwind fairly well at 12 kts. (There is usually a little bit of unfavourable current to factor in).
Interestingly...today I rode a Flysurfer Razor for the first time. Wow! that thing rips upwind like crazy. Other guys were out with 16m and 17m (latest greatest Cabrinha kites) and barely staying upwnd on their "fatty" twin tips. I ripped upwind so fast on the Razor on an 11m kite (depowered!!) I was amazed... http://flysurfer.com/produkte/boards/razor/
Now, in all fairness this Razor is a completely different experience to any twin tip or surfboard that I ride, It definitely took some getting used to and I still need to work on my transitions, etc...I ride upwind about a mile in 4 or 5 tacks...and when I decided I better go back downwind it was a bit of a challenge...so I need to work on that as well. Some people might not enjoy this sort of ride at all...I do because it is a new challenge, plus I was ripping in light wind with a small kite. I probably could have been on my 8m.
Sorry for the sidetrack, but the point I am trying to make is that...speaking for myself anyway...I probably would not want to ride this Flysurfer Razor all the time and I enjoy riding different boards , but mostly my strapless Lost Fish or 5/10 Firewire Taj, and when there are waves with light wind I usually try to make the fish work with a 15m kite (Core 15m XR2LW which turns pretty fast and can be really fun in waves in spite of what people may think). So you have to decide what style you want..if it's freestyle you probably want to look at a wider board with less rocker and some flex, 135-140 long x 43-45 wide
As said before by others you need a lot of power with a small TT in light wind, and when it dips a little below12 kts it sort of sucks...whereas a good light wind board (whether a TT or directional) will keep on flying for you...which allows you to get the best performance out of the kite as well. If you can't get a little apparent wind going you're already behind the curve. If you have a board that will get up and "go" quickly you can build/maintain that apparent wind...which is rather critical in light wind.
sorry for the ramble