I have the 5.15 update and have no issues with the brightness. My dad has an iFly that hasn't been updated, and is using version 4.XX. The brightness compared to my unit using version 5.15 is similar.
Besides the known bug (that I've never seen in 5.15) where cycling between day/night mode might not give you full brightness in day mode, I don't understand how any software change can affect the absolute performance of the screen. Once you crank up the backlight all the way, that's it. I think there are so many variables involved in the flight environment, that it's hard to compare screen brightness based on memory.
This brightness issue is purely subjective and I'm not discounting any of the posts on this thread! I just want to make sure that anyone thinking of buying an iFly doesn't hesitate because of any brightness issue that may be the exception and not the rule.
Also, I prefer the way the night mode dims, and hope it stays the way it is. Anything brighter would blind you in the cockpit if you're actually using it at night.