You can make 15 knots? You got me beat. I am happy to maintain anything over 5 knots.
When you are selecting a yak, you also need to consider where you will be using it. A 14-foot yak will be unmanageable on a Class III+ river. You need a short (9-10 foot), maneuverable boat, that will not bend in half if you get trapped between rocks.
Inflatables are a good compromise. My Advanced Elements Convertable handles rivers, fast water, bays, lakes and even swamps equally well, even though it is 15' long, and I can keep up with hard-shell Touring yaks in it easily (with the backbone installed). With the High-Pressure Dropstitch floor in it, I can maneuver in rivers as good as I can in my hard-shell Pursuit, because it gives it a flat bottom.
It's better to buy more boat than you think you will need, than not have enough. Anyone can get caught by surprise with sudden storms, wind, fast water, etc...