I shoot with a Canon Xsi and also have the Canon 100 - 400 IS that I purchased with advice from the forum. I originally purchased the 1.4x and tried it out a bit. I ended up sending it back and ordering the 2x. My main reason had to do with the fact that neither will work in the autofocus mode and I was struggling to get tack sharp images in manual focus. I found that the additional magnification of the 2x allowed me to focus better and as they are the exact same price, I could see no down side.
This photo was taken with the 2x in place and in cloudy conditions, but it gives you the general idea.http://s267.photobucket.com/albums/ii31 ... P_0001.jpg
Sorry, all I can offer is the link as I do not have the photos on this computer. I can't help much with the Tamron question, but I can say that the Canon does a really nice job and if you go Canon, go 2x in my opinion.