This chart shows 100% center crops from a chimney I used as a target. The first column shows the 500/4 bare shots, resized to 2x, to simulate twice the focal length.

The second and third shots are again center crops, first with a Sigma 2xTC and then with a Bower 2xTC. The Bower seems to outperform the Sigma in this test and other tests I made.

The fourth column shows the results with stacked TCs, resized to 0,5x to match the size. It appears there is little gain, however I currently attribute this to the Sigma TC that is going in first.

All shots are from wide open (Top crop in each column) to fully stopped down (bottom crop in each column).