How much better is the signature series? well I hate to be vague but optics are really hard to quantify. I see you have grabbed onto the light transmission issue, and while it seems like a good measuring stick for optics, it is really only one part of the picture. There are issues like resolution, sharpness, and contrast, all of which are affected by the lens quality. However, how much resolution, sharpness, contrast do you need for a hunting scope? Some would have you believe that you have to have the best for it to be a good scope, however, I' m a little more practical. I want a scope that is good optically, but most importantly it is reliable, and tracks well (meaning that when I adjust it 2 clicks right it moves 2 clicks right, not one click), and has sufficient eye relief for the gun it' s going on. I' m not advocating going to a cheap scope, because optical quality makes it nice. But, some differences are too small to be detected with the human eye.
So I went through all that to say this, any of the scopes mentioned have good optical quality, any differences between them may be difficult to see with the naked eye. However, you asked how much better is the signature series was and I can only say it appears to be slightly better that the Fullfield II, but not much. Additionally when you get to the Signature price range there are scopes that I greatly prefer to the Burris, such as the Zeiss Conquest (a great buy in the 3-9 by the way) and the Leupold Vari-XIII' s. I apologize for this essay on scopes and hope I didn' t go way overboard (I tried to keep it short, but my brain wouldn' t let me).
Finally, whatever scope you decide on I' ve got to recommend taking a look at
www.theopticzone.com I had a great experience purchasing from them, and Jon Jackoviak is usually hanging around and participating in these forums so it' s nice to support him.
Gordon