I think that would be very hard to say as many of the people that wear kilts still buy kilts from Scottland exclusivly.
With that being said I still would agree that the market is strong and that it is growing. If not only do to people wanting to wear kilts. In todays times many peoplee feel the needs to get intouch with there roots and understand where there "people" came from. I am sure this can be seen at Celtic/Scottish Festivals and the increse in attendance. Also the Female version of the kilt is very popular in young girls, well atleast in the South east i have noticed.