Don't know that I can talk you out of it. I have one and rather give up my 600mm II than the 300mm II. It is just a very versatile lens by itself and with both 1.4x and 2x extenders. If Canon comes out with a 7D II with 1Dx like AF, then it gets even more versatile.
Here are some samples:
300mm II + 2x III on 1Dx; f/8 ISO 1250:
300mm II + 2x III on 1Dx; f/8 ISO 1600:
300mm II on 5D III; f/2.8 ISO 100:
Let me know what you decide.
Thanks jasonsim for showing me your "bad photos"
I tried this lens at local camera store twice(without 2x TC III), I love the AF speed.
After seeing your photos, I'm 1-step closer to pull trigger now