Ok well it looks like no one actually wants to discuss this.

Personally I think it depends on the riding style...a spinner doesn't need a stiff frame and probably doesn't notice it at all. Personally when I get on a Cannondale I can feel the difference...hell I can even feel the difference between my Roubaix and my Merckx...the Merckx is stiffer for sure (same wheels/tires). On the Merckx or the Cannondale when I get up and hammer it feels like all of that power is going to the ground...on the Roubaix it doesn't feel the same at all. Now the Roubaix is comfortable as all getout and if I am spinning away it just doesn't matter...but when I stand or mash I can feel it.

Now is it really faster? I am not sure...but the last test ride I took on a C'Dale gave me a best time for a standard loop. Was it good legs that day or did the bike play a roll? Now if it was a ride I had done a couple of times I might be inclined to think it was good legs and high from being on the new bike...but I have done this loop a hundred times or more and on several different bikes...the Cannondale was the fastest by several min and nearly 1 mph in avg speed. It didn't make me a racer but I do feel it made me faster.