Canon 70-200 f/4 L IS with 1.4x or Canon 70-300 f/4-5.6 L IS or 300 f/4 L IS over Tamron 70-300 VC?

dheerc wrote:

Hi,

I bought a Tamron 70-300mm f/4-5.6 SP VC Di towards the end of last year and I have been shooting water birds and small birds in general on my Canon EOS Rebel T3 (1100D) for quite some time.
Recently, I rented the Canon 70-200mm F/4L IS and I must admit that it has been a pleasure to use it and the IQ is simply splendid in its focal range. I really feel like ditching my Tamron for it but then knowing that whenever I use these lenses, I always zoom out to the 200 or 300mm (Canon and Tamron respectively) whenever I shoot, I really feel like getting the 70-200 now wouldn't be a great idea so, I thought of getting the f/4 L IS with the 1.4x TC. This was when I noticed the 70-300mm L lens. I did some internet research and some DxO mark pixel mapping and Optical sharpness on an APS-C camera and I found that the 70-300 L has a similar map as my Tamron, a little better for sure but not as good as the 70-200 f/4 L IS. During my research I also came across the 300mm F/4 L IS as well and thought that was attractive for its price but then again, weight considerations come in.
Weight wise the 70-200mm f/4 L IS is identical to my current Tamron but quality wise, they are very different! At 300mm, the IQ of the Tamron is poor; even in the overlapping range the Canon is hands down the winner.
So, my dilemma now is, which lens do I upgrade to? A 70-200mm F/4L IS with 1.4x (used copies of both are pretty attractive for their price on some forums!) converted to get me to about 280mm and then later crop the image ( which by the way, I must say, without the converter now, look amazingly detailed!)
I got this image at the 200mm end and cropped it to around 50-60% to get this:

I love the quality of this lens but then again, if the 280mm with the teleconverter is going to be as bad as my current Tamron, it isn't a worthy upgrade. On the other hand, the 70-300mm f/4-5.6 L IS's IQ looks almost identical but I did see that it became a little soft at 300mm. The 300mm f/4 L IS lens's IQ looks pretty fine, in my opinion but it's huge to carry around!
What are your opinions and suggestions in this regard ? Which one would you prefer with similar conditions that I have? I don't want to make the same mistake as I did with my Tamron again.
Thank you for your time and constructive comments.

The Tamron is very good @ 300mm f11, and it can even be good @ 300mm f5.6 if you do the trick I posted in the other thread:

http://www.dpreview.com/forums/post/56334662