sigma 50-500mm seccy hand or canon 100-400 new , sigma also do a 2x adaptor was looking to give that a try too
The Sigma 50-500 does have a massive range, and i think they also do a 150-500 which I've heard gives slightly better quality results.
I've just got the Canon 100-400, and it's a very well built piece of kit, and I actually quite like the push-pull zoom. Haven't had time to use it properly yet though
Teleconverters are okay, but you generally lose a stop or two and they can slow the autocus down (if it will still work with an extender, which it may not depending on