Dave,


I went through a similar dilemma. I had (have) a Tamron 70-300mm and felt like I needed more reach. I had considered what Fast Glass had offered as a suggestion, the 100-400mm L. Ultimately I decided to keep my Tamron and purchased the Canon 400 f/5.6L prime in stead.


I never touch my Tamron anymore. I was in love with it, but have recently noticed how soft it gets at the long end of the focal range. Although I know with a certainty that my 400mm is a tad bit sharper than the 100-400mm @ 400mm, I now wish I had made that my choice. I can never bring myself to put the Tamron back on my camera.


So my advice, Don't get the 200mm with the 2x. Ditch the 55-250 and get the lens that I should have. (Still LOVE my 400, but the 100-400 would have been more practical.)