OBX is pretty windy and you already have an 18 C. Do you have any other kites? If not, why not get something like a 13 meter waroo that you can hold down in medium to higher winds? The 13 will be alot more fun than the 17 - and you will not give up that much on the low end.
Also, if you get or make a large flat 2nd board - you will get the same range out of the 13 /big board as you will out of the 17.
For the last two seasons my 20m Yarga was my " bread and butter" kite when I was riding in SoCal. When it finally died I picked up an '07 17m Waroo. I would say that it delivers the same if not more power as the 20m C but it feesl more like my 17m Yarga in the air. It definately goes upwind a lot better and when you're powered up and the hang time/ glide is insane.
It's really fun too if you just park it and use it for no whip stlye tricks. So far I've had no issues with relaunch.
I would highly recommend it as a light wind option for a bigger guy. I also have an '06 14m Waroo and while I can get going in the same winds on a bigger board it's just not as fun for me. It really bolis down to how you prefer to ride. I managed to hold it down to 20mph comfortably the other day and I still had some depower left on the strap. I would imagine 25 mph would be out of the comfort zone for me with the 17 ( I'm 200 lbs).
I had a 2006 Waroo 16m that I replaced with a 2007 Waroo 17m. I weigh 210 lbs and ride both flat water and surf. Comparing the two kites, the 17m has better low end and is a more grunty kite. Haven't had a chance to test the low end threshold, but my guess is that the 17m would get me going in 1 or 2 mph less wind than the old 16m. The 2007 17m also appears to have a more bomber type of material construction, especially the new leading edge stitching. From my perspective, those seem to be the extent of the advantages of the 2007 17m over the 2006 16m.
The 2006 16m was faster, lighter, and had much more range (top end) for me. I was able to ride the 16 up to 25 mph with no trim and an instructor friend of mine said he saw me out on the 16m in winds of 32 mph (if I really was, it was fully trimmed). Don't even think I can hold the 2007 17m down fully trimmed at 25 mph - will have to try that soon.
I got the 17 because there were a few light days in the surf where the old 16m couldn't get me going. That problem seems to be solved by the 17, but I miss the faster turning and greater range of the 2006 16m. Also don't care for all of the battens on the new kite. Not a great idea if you drop your kite in the washing machine.
At 205 pounds, it's a tough call between the 2007 17m and the 2007 15m Waroos. If you had a 164 Spleene Door, I would say get the 15 because the low end would be extended by the Door and the 15 would definitely have a better high end (meaning you wouldn't have to switch kites so soon). If you get a lot of 15 to 20 mph days and you're going to keep riding the smaller 138, then the 17m would probably be a better call. If you have a chance to demo both for your conditions, that would be the way to go.