The main season for sri lanka is May to September. During which the winds are fantastic. The primary place I go to is Kalpitiya to a place called Kite Kuda, although there are several competitors who do a fine job there.
January is a secondary winter season, which I have never personally experienced. That said the wind does look quite ok. It's not everyday like May to September, but it can be 3 days on 2 off or something like that. Winds can be very strong sometimes.
I don't know anything about the rest of the island, except that the northwest (kalpitiya) seems to have the best wind, by a mile.
Contact Mike at KiteKuda with questions. He's a friendly australian guy. Tell him "Tyler" told you to contact him. He's a good friend. Here's his school:
The regular gfs wind guru seems to predict wind conditions almost perfectly. Which is also very rare in asia. http://www.srilankakiteschool.com
(I am just a regular client; I have no business stake here)