Generally they match B&H if you push them, they say everything is 'duty free' but they just mean there is no sales tax, and most people don't pay sales tax on B&H purchases either. They try to do deals or bundles, ie cards and extra batteries and often have older models that you can get deals on.
Warranty is a mixed bag. Most times you get an International Warranty (purple card), which means you can get warranty service anywhere (Canon USA do work on International Warraty equipment, well they have for me), though sometimes it comes with a Canon Latin America Warranty card (brown card). St Thomas, as far as Canon are concerned, is not the USA it is Latin America.
It also depends a bit on what you are after, TS-E's etc forget it, 7D MkII's they should have plenty. I haven't been there for a little while and the other huge variable is the exchange rate fluctuations that are going on, I doubt it, but there may be a bargain or two around because of that variable.
Last thing, you have to haggle, hard, and play the various outlets off against each other, don't buy in the first shop you go in even if it is the best price, it will come down after you leave and come back and say the other place is throwing in a third battery and bigger card.
I know all this because for 12 years I lived next door to St Thomas in the BVI and it is where I bought $10,000's of Canon gear.
Personally I wouldn't bother, it is much more fun to get 'the Island tour' with the camera you already got from B&H without hassle, and stress free return policy, and you don't spend your excursion time battling sales people.
