One thing to consider since you're getting a 7DII is that with that camera you can add a 1.4X TC to your 100-400 and it will still auto-focus. That would put you very close to the 600mm. Since the Tamron is an f/6.3 lens, you would lose AF if you added a TC to it. IMHO, the 100-400 handles a TC very well and the only problem has been losing AF when using a TC. Here's a sample shot taken with a 100-400 and a Sigma 1.4X TC (manual focus).