They do lose tension over time. That's indicated by them being 'sprung'..not seating on the reedblock 'at rest'.
So, it takes back pressure by the piston to close them instead of them tending to close themselves. That effect is going to be more pronouced on the bottom end than the top just due to the speed of things happening.
The stiffer they are, the harder they are to open..but the easier they tend to shut. That would tend toward a good thing for hi-rpm operation.
Conversely, a reed that opens easier (but by that nature would tend to stay open, too) will tend toward enhancing the bottom end.
The more 'petal area' you have, the less they have to open (and flex) to get the same amount of 'air' through. The less the petal opens, the sooner it can get back to where it started (closed).
THIS sounds like a good workup for a combination of lo AND hi rpm power!
Enter the DFII!!!
:)