I just want to throw a wrench into this discussion from an amateur who can't even dream of owning one of these super-tele's (although I keep buying powerball tickets). This question is addressed to those who have been using the 300/2.8 with TC's. Last year, Sigma updated their 120-300/2.8 and the price is far below the Canon 300. Has any one of you given it serious consideration? Why or why not? (I know the Sigma 300-800/5.6 has some strong followers, so I assume it is not just because of an intrinsic bias against 3rd party lenses). I keep thinking it might be the only way some of us could afford to get an AF 600mm. Thanks for your thoughts.