Well, first report on my 2+1 Kona 11'5 in not-very-good conditions...
Yesterday I was on the SE shore of Lake Erie but the wind fizzled out before I arrived. Funny how that happens. Anyway, stuck too late in the day to go elsewhere, I rigged the 6.3 SF and the K 11'5. Wind was about 4 kts, waves about 15 cm. This situation is a major reason I got the K 11'5, because it happens that I take a long drive and end up with such conditions. The K 11'5 is a one-board solution for minimal conditions like this, and I also use it up to solid 6.3-7.0 conditions (when I would switch to a shortboard).
The fin configuration I tried is a Maui Fin Co Weed Wave San Carlos, 25 cm, with Hanalei Fin symmetrical 5" side bites (~12.5 cm). The toe-in on these fins is 1.5º, and the insert I have gives 8º of cant. In the past, I've used the Kona OEM fin as well as a True Ames Surfgrass (these are much larger, at the upper limit for a US Box). One thing I like about the US Box / Pro Box combo is the ability to move both the center fin and the side bites fore and aft. In planing conditions, I'm hopeful that this will help tweak maneuvering ability. If I had any "complaints" about the single-fin version, it was that the bigger fin sizes required real commitment to carve hard on cutbacks. Obviously, I didn't get to experiment much with that yesterday.
The waves outside were too weak to catch without some wind to help grab the wave. In some "gusts" I was able to get on, but most of the wave riding was deep inside, ~10 board lengths from shore. I can't say it seemed looser in this mush, but it did seem pretty easy to catch well-formed waves. On the short-coupled lake waves, there were times when I got the nose a little immersed, but she rode well and hasn't ever shown a tendency to go diving.
Exocet adverts this board for up to 9.0, and I've ridden it with my 8.0 SF and it's really not that good. Heavy feel, not at all like she is with a small sail in the gliding mode. IMO, better to put a 5.3 - 6.3 on it, and surf the inside. One thing I really like about this board is that, in contrast to other windsurfing longboards (here I mean ones with a centerboard and not SUPs with a mast track), the K 11'5 really stimulates you to stay in the shore break / surf. It's floaty, easy to tack even in the whitewash, lifts over the waves when you're underpowered going out, and will catch dinky little waves. It's not hard to stay dry on the K 11'5. Gives you something interesting to do when you get there and the wind dies.
Once planing in waves with 6.0-7.0 wind, the straps are in the right places and it's a decent ride. And it FEELS like a windsurfing shortboard because (putting in the fin boxes I found out) it's full sandwich construction. I don't think I would jump it, but it feels very solid when planing or pounding into a wave. I'll follow-up when I've had a chance to ride the 2+1 configuration in the planing mode.
So for folks who live in areas with small mushy onshore conditions, I find the 11'5 or 10'5 are a good choice for the range of minimal-wind to moderate conditions. The way I look at weather maps, in most areas of the world, that is the most common weather. So it seems like a no-brainer to have something that works in that wind range. The options are:
1) a SAIL board that you will just sail, and a shortboard,
2) an SUP (you probably won't sail), a transition board to sail as well as a shortboard.
I want to sail more than surf because I find part of the magic is in the sail, so I'm not interested in the SUP. I can see that surfers might want the SUP solution when there's no wind at all, but the surf is good and they want to ride.
Solution #2 is probably the most versatile, but #1 works well for those who need a smaller board quiver. The Kona's (Curves) work well in that spectrum.