Don't get the TC, swap for the Sigma as you said. For starters, a canon TC almost certain;y won mount on the lens, which is a clear indication that you shouldn't mount a TC in the first place. The next problem is that you most liekly wont have any Autofocus, and even if you used one fo thelateta bodies that does support f/8 AF then you will fin it incredibly slow. Plus the cost in IQ is preetty big and only worth it if the lens is outresolving the sensor, which in this case it isn't
The Sigma 150-600 C is actually sharper than the Canon 100-400 over the shared focal range so you don't give up anything.