Updated with more testing. Added to bottom of this post.
Sigma 150-600 Sports vs EF300 F4L IS with Sigma APO 2x TC vs EF100-400II with EF 1.4xTC III.
Multiple shots taken with 1DS2, 1/500 sec / f8.
Refocused for each shot but found very good AF consistency for all 3 lenses.
Full frame then a 100% crop.
I'd give Sigma 1st place, maybe expected as it was the only lens stopped down and only lens used without TC.
Relevant IMHO as purpose of test was to find the best option for around 600mm ( Edit: and only results at 600mm ) without taking a huge financial leap to a super tele.
More testing. This time using a "proper" test chart, indoors.
Canon 100-400II with 1.4TCIII and Sigma 150-600 S.
Distance to chart 8 metres. 1D4 ISO400 f8 1/180 sec.
Live view focus, IS and OS off, beanbag and remote shutter.
Lit by 85W CFL.
RAW converted to JPG in DPP4.