I doubt you'll lose 40fps an inch, or, if you did, whether that would matter at all. If I recall correctly, Mule Deer wrote an article, or maybe a post, comparing a 24" CZ 550 to the 20.5" CZ 550 and the shorter barrel actually gave slightly higher velocities with some loads.
I wouldn't obsess about the shorter barrel.
To the OP: if you're really worried about the shorter barrel, get the 24" version so you can sleep at night. Perhaps there really is a practical difference. Perhaps there is also a difference between 10 and 15 Angels dancing on the head of a pin
