Tonight I did some more tests with this lens, at f5.6.
Also, I tried the external aperture.
The results (crops from corners) are attached.
While CA is much better at f5.6 with internal iris, it is still barely visible.
However, with external aperture (17mm dia, which makes F5.6) it is gone.. only to be replaced by coma. So, all I can say is this lens is pretty well optimised for what it is...
While I am not particularly happy with that amount of CA with internal iris, I discovered that I can get rid of it almost completely with software
Gimp has plugin that deals with this nuisance.
It is sort of cheating.. but I will have to live with it, it seems.
All in all, good lens!