The Kenko 1.4X TC does not work on recent Canon cameras for certain lenses, I've not seen a list yet for the 7D MK II, or seen anything on the forums about issues with this lens plus a TC on a 5D MK II.
You may be better off going for the Tamron 150-600, as far as getting a longer focal length.
About all you can do with regard to the Kenko, is to buy one somewhere that accepts returns in the event it does not work.
As noted, TC's work best on a sharp lens, and magnify any flaws, so that might be a consideration.
Another possibility in the low price range is a good copy of the 100-400mm L MK I. It accepts TC's and is good with them.