I cannot speak specifically for the Fly as I have not owned one, but I owned the Ixon IQ, which I understood to be the battery powered version of the Fly -
http://www.bumm.de/index-e.html?docu/192q-e.htm
They also rate it at 40 lux.
I also owned a Cyo (well I still own a Cyo).
As I mentioned, I do like the Cyo, my two complaints are just that it isn't quite bright enough for the worst terrain/conditions (black pavement) when it's raining (again, in my opinion) and that it doesn't throw a lot of light off to the sides.
But - I really like the light and I've kept it on my commuter bike. And you don't know me, but believe me, if it wasn't bright enough I was have returned it or sold it by now. :-D
In contrast, I used the Ixon IQ on 3 rides I think before getting rid of it. There were to many times I just felt like it didn't put out enough light - on the exact same route I rode with my Cyo, and with no rain or water on the path/road at all. It just wasn't as bright.
So assuming that the Ixon IQ is identical in it's light output to the Fly (like it's supposed to be), there was no doubt in my mind that the Cyo was noticeably brighter.
Again, these things are my opinion, and come with no warranty expressed or implied (lol!).
If you're happy with the Fly, there may not be any reason to upgrade...it's up to you.