I thought that the process of dark-firing tobacco increased the amount of nicotine in the tobacco (or at least the concentration of nicotine in any volume of leaf).
I don't think firing the tobacco has any effect on the nicotine levels, but may have some effect on the sugar content. I don't claim to be a scientific expert on the subject by any means. But I can say that the stuff is packed full of nicotine, so much so that you can get a buzz just by handling the raw plants.
Dark fired tobacco is actually a strain of tobacco, not just the same ol' burley put through a different curing process. It's very high in nicotine content, and I don't think the fact that it's fire cured has any real effect on that nicotine levels. I think it's more about flavor honestly ... but again I'm not a scientific expert, I just grew up on a dark fired tobacco farm
Also, dark fired tobacco has been around since long before the february 2007 "grandfather" date (that the house is attempting to amend to a later date). I think the FDA regulations are more about how it's processed by the manufacturers than how it's cured by the farmers. Tobacco curing hasn't changed very much in the last couple hundred years. It's either air cured, flue cured, or fire cured. That's about all the options you have in curing a tobacco plant. Now aging, on the other hand. That's a different story.
Here's a handy dark-fired guide from the University of Kentucky:
http://www2.ca.uky.edu/agcomm/pubs/agr/agr152/agr152.pdf