I should blinking well hope so at that price!You can get some great shots with a 300f2.8 using a 2X converter.
interesting feedback guys the 800mm is available at £3566 and I was thinking of this intead of a canon prime which are a lot more with less reach.
I added the sigma 500 f4.5 to my 50D 100-400 kit. I am still finding this a big leap in how I am thinking about taking photographs especially regarding mobility and fieldcraft. I'd need a load more experience before I thought about 800mm.
Good Luck though!
Buy it's a big heavy lens, the length of it makes it very trocky to use from a car or in a small hide.