Wow, this got a lot more interest and replies than I thought. Thanks for everyone who took the time to answer, a lot of great advice was offered.
I saw a bunch of recommendations to invest in the canon f2.8 ii IS. While I have no doubts it's a fantastic lens, I can't help but make the comparrision to when I was buying my camera body a few months back. I was looking at a 6D vs. a 5Dm3 and also got a lot of ''get the 5D, it's the best'' (excluding the 1DX) and many people saying I'll regret the AF system in the 6D. But I got the 6D (for 1400$ new), and I'm thrilled with it. It takes stunning pictures, the AF has 99% been flawless and the few times it hasn't, I've made adjustments and got on with it. I borrowed a friends 5D for a day and while it's a better camera, I can't help but feel I got 90% of the camera for half the price.
Point of that was to say I don't feel I need the best of the best, I'm more looking for excellent quality without resorting to robbing a bank. Therefore I feel I just can't justify the extra money the canon will cost me. If I'm going down the used or refurbished (something I did for my other lens and I'll gladly do again), the tamron offers me savings of at least 600$.
As for the other options mentioned, the canon f4 IS looks fantastic, my only concern is that if i decide to attach a 2X converter, I'd loose autofocus. The converter would be a last option, but it would be nice to have that reach if I so decide. I'm also concerned about people posting that the m1 version of the 2.8 IS has much worse IQ. Can Tamron and Sigma really not make a lens in the same ballpark as canon? Both the sigma and tamron are newish lens as well.
Again, thanks for all the great replies, keep them coming.