I'm a lefty and I use my mouse with my left hand always have been. And yes, it's often been a pita with lots of mouses so I had to be careful when buying one. Currently I'm using a logitech mx revolution: despite being 'optimised' for the right hand, it's quite easy to find a comfortable grip with the left hand even though there are buttons I just cannot use since I can't reach them. I absolutely cannot use efficiently my right hand with a mouse so yes I'm definitely interested in a Naga for lefties.
While it's true that a lot of lefties use the mouse with their wrong hand, AFAIK most of them do it because the first time they sat before a computer the mouse was on the right and they assumed they had no choice in the matter. Seems I've got lucky since I've bought my first mouse myself and no one decided for me where it should be relatively to the computer...
IMHO every lefty using the mouse with their right hand should try real and hard to use it with their natural strong hand. So far all the people I've managed to convince to do so have been happy with it and never looked back.
It occurs to me that mouses for lefties are like scissors. With effort you can be pretty good at using regular scissors with your left hand but chances are big that you'll never been as good as a right handed person. Personally, I've never been good at it and for years I've accepted what everyone (parents, teachers, friends, ...) since childhood was telling me: that I was hopelessly clumsy. Up until the day my wife bought me scissors for lefties and I realised a simple truth: I was never clumsy, it's just that with regular scissors I simply couldn't SEE what I was cutting...