Given that this lens and the Canon 24-70 L II are both coming out this month I would just return that lens anyway. In 4 weeks, once we all know more about both these two new 24-70's, you will either want this Tamron new or there will be a glut of used 24-70 L's on the market as people trade up to the II and you can grab a used one in excellent condition for considerably less than $1329.
Thanks for the advice. I'm getting married April 29th and I need the 24-70 for that day. That still leaves me 5 days to return the Canon.