Iefan,
a few points.
Are there any negatives to the larger Fields of View? I.e. 60 Degree Field of View offered by Helimun? Why would one want a 40 degree Field of View if you could have 60 Degrees?
The FOV is increased by the optics, so 1.5x wider FOV means also an image resolution (in lp/mm) decreased 1.5 times. A point usually omitted.
Most of the questions you are asking are very technical. Even getting the answers will not help you a lot, as the parameters you are asking for are measured at a given lighting level. When that changes (brighter or darker), the parameters are also changing. The question is how.
As crabb pointed out, IITs are just a part of the problem. There is also weight, mount, etc...
One more point. You should be able to get XR-5 with gated power supply, whereas
AFAIK US made gated gen III IITs are ITAR restricted.
Gen III IITs will perform better in low light condition than XR-5.
Last point to consider is of course cockpit NVG compatibility. The IIT choice may have an impact here.
Arrakis