I don't know anything about tin paint and who or what it is bad for or even if it is. But, let's assume the paint job does last 5 years. Where will you be in 5 years? Can you over coat the tin paint with something else? If not, you'll need to strip it if you can't get it done again with tin paint.
I'm not an expert and there are people here who know more than I, but I have been reading quite a lot about bottom paint lately. You should be able to get several seasons out of a good paint like micron or one of the Pettit paints, with a little touch up each year. If you really need three gallons that's about $450 for the paint. A 40 foot boat would cost about $1,200 for the labor, so a total of $1,650.
It still seems like a better deal to get it here, but even if it costs a bit more, at least you won't have to worry about how to over coat it when it is ready. If you ever plan to sell the boat, it would not be right if the paint is illegal without at least disclosing that to the buyer.
So why look for hassles? Regardless of whether the tin paint is really bad for anything, it just seems like using it could bring on some problems in the future.
I can't resist one comment though. Like I said, I don't know anything about tin. But I do know that DDT was also banned worldwide, and that ban has caused many deaths in developing nations. The ban has killed many more people than it is claimed to have saved, and any evidence that people were saved by that ban is sketchy at best.