i have been on the fence about buying that lens, i would be using it with the tc alot. i have looked at the test shots on tdp. it looks to be sharper than the older (non sports) model until you put a tc on it and then it looks worse? maybe that is some kind of fluke but that is what is holding me back. maybe you could take it to a store that stocks canon, sigma, and kenko tc's. i am sure they would let you take some test shots and let us know what you find out?