After testing close to two dozen lens combinations the Sigma 150 OS macro + Xenon 28/2 was one of the top 3 lenses. This image quality that this lens combination produces is better than any lens I have tested and I found that you would have to go to a high-end objective to beat.
The Xenon f2 lenses are not as popular as the later Componon lenses but I have found these to be much better performers than the 28mm and 35mm Componons.
STACKED VS EXTENSION
Stacked or coupled lenses are a pair of lenses mounted face to face for macro photography. The performance from a pair of stacked lenses at 1x or higher can be much better than a normal macro lens focused using extension. A f/3.5 lens focused at 5x by extension only would have an effective aperture of f/21 but when you focus at 5x with that lens reverse mounted on a telephoto, the effective aperture would be f/17.5. That gain of 2/3rds of a stop can make a big difference in image sharpness.
MOUNTING ADAPTERS
This setup was made using adapter that I had on the shelf, a 72mm to 52mm step-down adapter, a 52mm to 42mm step-down, and a male 42mm to male 30.5mm adapter to mount the Xenon in reverse.
To decrease the distance between the two lenses I would like to eliminate one of the adapters that I used in the above photo. But when I ordered a 52mm to 30.5mm adapter ordered from China, the adapter turned out to have the wrong thread pitch so it was basically a throw-away. Cheap parts from China have become so hit and miss I am thinking it might be better in the future to just custom order every adapter to save time. It seems like about 50% of the adapters from China are not what was promised and end up in the garbage making these not so much of a bargain in the long run.
Image quality at 5x
To see how the Sigma 150/2.8 OS macro + Xenon 28/2 lens performs at 5x against 10 other lenses follow this link:
https://www.closeuphotography.com/5x-lens-test
Other Stacked Lens Combinations
The Xenon is also a great performer at 4x on the Schneider Makro-Symmar 120/5.6 lens, you can see more information here:
The Schneider Makro-Symmar 120/5.6 + Xenon 28/2.8 Line Scan Lens performance is extraordinary and best the best 4x microscope objectives that I own, you can read all the details here on Closeuphotography.com: https://www.closeuphotography.com/stacked-vs-objectives
LINKS FOR MORE INFORMATION
Sigma 150 OS macro lens information on the Sigma USA website:
https://www.sigmaphoto.com/150mm-f2-8-ex-dg-os-hsm-apo-macro
Xenon f/2 lens discussion on Photomacrography.net:
This is an older discussion but there is still interesting with lots of info.
http://www.photomacrography.net/forum/viewtopic.php?t=7354&