It depends on when (in the year) you want to go, and whether you want to go to a clinic, or simply go to a very nice spot that provides lessons and rental gear.
In the winter, at your suggested skill level, Bonaire, in the Caribbean could be just what you are looking for. You won't find the deluxe accomodations right at the sailing site, but if you rent a car or a scooter (not much $$) you can have the deluxe accomodations and great dining in town. If you decide on Bonaire, check with Annie Phelan at http://www.bonairecaribbean.com/
. Annie can arrange almost anything you can imagine on the Island and will greet you personally (most of the time), when you arrive.
But you need to decide what skills you are looking to improve.
Select an instructional program that suits your specific needs.
If you goal is to sail better in your suggested 10-15+ knot conditions, look for a site that gives you similar conditions.
Going to a "hi wind destination" might improve your skills in higher winds and on smaller boards and sails, but will do very little to improve your skills on larger boards and rigs in your stated conditions.
Too much wind could be intimidating, and while exciting, having conditions beyond your "comfort zone" may result in you not getting out on the water enough to work on the skills you need.
If you decide to take a "clinic" you may find one of two things.
The skill level "intent" of the clinic is swomewhat above what you are comfortable with and you won't "fit in" with the rest of the students in the clinic.
You may find that the instructional program does not match well with your current "skill set" and the clinic will be about changing "how you sail" completely. This might be good, and it might not.
If you try Bonaire, the instructors at "The Bonaire Windsurf Place" (a Starboard center, by the way with great rental giear for all skill levels from Starts and GO's to the small Carves) can individualize your instruction to suit your skills.
Lac Bay is shallow, beautiful with miles of white sand bottom about knee to waist deep with some deeper "green water" conditions down by the channel. The wind is predominantly side onshore and makes sailing in Lac Bay super safe.
Florida (Calema) might be OK, but it will be colder and the wind will depend on the weather fronts coming through in the winter, and it may be very hot and humid with very little wind in the summer.
Maui would be good, but it's very windy and the conditions are rougher, even in close to shore at Kanaha.
Other Caribbean Islands (Aruba, Cabarete, etc, could be good also, but they generally have offshore winds which are a bit more intimidating for progressing sailors.
I'll be in Bonaire Jan. 21st-Feb. 10 2007.
Hope to see you there.