Moving from a Sigma 150-500 to Canon 300mm f4 + 1.4x TC combo - makes sense?

This is a switch I am contemplating as well; but I am really tempted by the Sigma 120-300 2.8 with Tc's

Assuming shooting birds. Might as well as get 400mm f5.6. 300mm f4 with 1.4xTC, no in my opinion. Lot of folks use it and recommend it. I will glue 1.4xTC on 300mm f2.8 IS not on f4 IS.

Based on the feedback so far I'm going to give the 400 f5.6 a try. Thanks guys!

Thank you Samsen - hoping my 55-250 would help fill that zoom need.

Thank you Kenji...I primarily shoot birds with my super teles and I'm hoping the details would be good enough to crop large and maybe help bring down the reach loss I'm going to have.
The current Bigmos I have sure is a good copy and for all I know might end up keeping it too

Morris - have heard a lot of good things about the 2.8 zoom too, but I think I'm going to stick to a prime.

Bobbyz - thank you! As I mentioned earlier hoping the IQ makes up for the reach loss (if any due to the focus breathing that would happen)