I've been using the Canon 55-250mm lens for a little over two years. I have not heard of a good 2x teleconverter for that lens. So, what I do is challenge myself to get closer to the wildlife. We were discussing it the other day, that you could buy or make a bird screen. It's basically a tent with holes in it for the camera etc. (to keep you hidden from them). I don't know if that is why you want more zoom, but doing things like that can help and it doesn't cost a lot of money. You could save up for the Tamron 150-600mm lens, as soon as it's available. That's a good option too.
|Common Flashwing by digimania|
from canon dslr
|The Marilyn impersonator by Lee8282|
from Blowing in the Wind (Nature)