Thanks for the input Senna4ever. I'm still thinking about it, but I think I'll probably just stick with the Tamron for now. I was going to buy a used one from a guy on POTN who was in Canada for $1000CDN. It didn't really feel like a "killer deal" that's cheap enough to justify me ditching the Tamron.
Or...alternately, I can spend it on something else...I've been eyeballing some ultra wide angle lens (Canon 10-22 or Sigma 10-20) that would probably be more useful.