fox1 wrote:Which would turn the 70-200 into a 100-340mm? forgetting the
1.6 factor? Forgive my maths...

More like 98-280.
Many, including myself, would claim that the 70-200/4L with a 1.4TC is superior (optically) to any of the xx-300 cheap zooms.
If you want to spend big bucks, then the 100-400/5.6L-IS is a good candidate. It does, afterall, have a longer reach. If the sigma is "close" in price, then I would stick with Canon.
Another (expensive) option is a 300/4L-IS with 1.4 TC. 
My biases: I had the 75-300. Upgraded to 70-200/4L. Found it to be excellent. I am not yet limited on the long end, but I am eyeing both the 1.4TC and 300/4L-IS to extend my reach further. Since I already have the 70-200/4L, I feel the 100-400/5.6L-IS would be a bit redundant and the 300 prime would be "better"
Your milage may vary.