I do almost exclusively concert photography. I have been getting amazing results from my Tamron 2.8 zoom lens, but I need a telephoto at this point. I purchased a used Sigma 50-150mm APO EX DC and love the lens, but it's too slow for my purposes and I can't use it. (I'm now offering it for a really good price in the classifieds if anyone's interested). What do you recommend? (other than a VR telephoto, because honestly, I can't afford one of those right now, besides the fact that as far as I know, most concert photographers don't find the VR to be particularly useful). Thanks.