You won't lose any stiffness unless you go through multiple thousands cycles (flexes) assuming you don't abuse the racket (leave it in your 200+ degree trunk for months or throw it in an oven. ) Even then, any loss of stiffness would probably be rather slight.
Light, stiff rackets suck IMHO.
If you play against hard hitters, there is no substitute for mass. It is simple physics.
I think you have to look at any manufacturer's claims with a jaundiced eye, youtek or otherwise. Not buying the racket stiffness changes due to your swing speed thing at all myself.