I have a new Sigma 70-300 APO DG Tele/Macro Zoom lens and I am enjoying exploring its possibilities, but for some things I'd like to try it just doesn't have enough reach. I can't afford a bigger lens so I've been looking at TC's, 1.4x in particular. These are within reach for me but I wonder about any trade-offs they bring in terms of IQ, AF issues or anything else. Is there anyone able to give me some feedback on these, particularly in combo with my lovely new Siggy or similar lens? TIA.