I bought the Sigma 1.4x TC to try with the 150-600S right before my trip to Florida.

It does impact IQ and AF...but, I can see using it in certain instances.

Canon 5DIII, Sigma 150-600S with Sigma 1.4xTC, @ 840 mm, 1/1260, f/11, ISO 1600

small-5044 by kayaker72, on Flickr