I had a chance to try out my IRIX on a night sky recently using the Astrotracer function on my K1. "Seeing" was not ideal due to an onshore sea breeze but I'll be quite happy to continue using the lens for this purpose.
* My copy of the lens seems nicely calibrated at the factory for infinity.
* Stars over the majority of the fov are nice point forms. There seems to be some coma near the right edge but it's pretty minor, and some star trailing is evident away from the centre due to the very wide fov.
* Constellation shapes are distorted towards the edges.
* Colour rendering is quite acceptable to me with stars like Aldebaran and Betelgeuse appearing as they should, although a very slight shift in colour towards the red end may be warranted.
* I found that f2.8 at ISO 800 to 1600 captured plenty of stars with good contrast.
I've included four images below; not the best "seeing" as I've noted earlier, but a happy start with the IRIX. (Notes on Flickr).