Ok, lots of debating going on here!

My original plan ...

Keep: Canon 135mm, 100mm IS macro, Sigma 85mm, Sigma 17-50mm
Sell: Canon 100-400mm, 70-200mm f4 IS, 200mm f2.8 II, 50mm 1.8 II
Buy: Used 70-200mm f2.8 IS II & used 2x III

Now, after seeing the post that the Sigma 50-150mm f2.8 EX DC OS is available I am comtemplating this idea ...

Keep: Canon 100-400mm and others mentioned above
Sell: Canon 70-200mm f4 IS, 200mm 2.8 II & 50mm 1.8 II
Buy: Sigma 50-150mm f2.8 EX DC OS

Second scenerio would give me more $ in my pocket in the end and still allow me to a variety of scenerios (birds, portraits, fast moving dogs & grandkids). If worse comes to worse, I can always sell the 135mm &/or 85mm down the road and still be well covered lengthwise just not so much for low light.

Questions:

If at some point, I still decide to get an extender, are the new Canon extenders compatible with the Sigma? If they are, would it be better IQ to use a Sigma extender on a Sigma lens? Anyone here use the Sigma extenders?

Opinions on Option #1 vs. Option #2 welcomed especially opinions on the Sigma 50-150mm IQ.

Thanks Much!
Denise