I have an opportunity to pick up a Sigma 300-800 for a good price. I've been saving for a while to get a long prime for when my 300 2.8 + 2x TC isn't enough. I'd be using it for wildlife and some field sports. I'm interested in hearing thoughts from anyone who has used or currently uses this lens. I'm specifically interested in how well it will take a 1.4 TC and how well the AF performs. I realize that this isn't a 600 f/4 IS or Canon 800 f5.6 IS but I'd really like to know if it will make a decent poor man's super tele, at least until I hit the powerball 