Yes, of course. But can you tell the difference between Project X 5.5 and 6.0? Or any other shaft in stiff and regular? Or even the same shaft stepped? Crossfield argues that flex is not important at all. Yet fitters and golf club manufacturers argue that it does matter. Unless you set up a blind study with tons of data, the actual effect of shaft on ball flight cannot be determined.
I think he is more arguing, in the numbers there is no significant difference between flexes. He tested out two different shafts, that had a good difference in flex. I will say that I think a more solid test would be to take the lowest flex and highest flex in the same shaft model. It's tough to compare two different shafts because their bend profiles could be different. This is why the lettering on the club really means nothing at all because a stiff flex in one brand isn't the same as another. If you have a bend profile that is overly softer than another shaft, then their stiffer flex might be another brand/model's regular flex.
What I take from this is, as Mark says numerous times, get fitted. Go see which combination works best for you in feel and numbers.
I agree, if you take Project X, and go down half a flex, you'll be hard pressed to tell the difference.
Maybe I'll go to golfsmith this weekend. I'll demo some clubs, and hits some shots with different shaft models and flexes. I'll take pictures of the numbers and write down some thoughts on it.
Honestly I don't care about what flex the club is, I'm just going to hit golf balls.