I had really bad roof flex, then I put wood in it. Hmm...
There is another dimension of flex that most people don't account for. Basically people (like customers) like flex because they correlate it to how much "power" the system has.
From a business standpoint (for the most part), flex is good.
From an audio standpoint, extreme flex *****. I think minor amounts of flex may actually help with the overall sound. But when a panel is flexing like 2", all the energy being used to flex that panel isn't being used to produce sound in the vehicle, just the movement of the panel. I got louder to the ear substantially when I brace my roof. It flexes maybe .25-.5" now where it moved 1-2" before. The response is much smoother as well. If you want to sound better and be louder, flex is your enemy.