All of the "r" numbers are made up to show the difference between the higher priced and lower priced stuff. Which is fine, but it's not based on any "real" characteristic of the bike. For example, back in the day the difference between 6r and 8r on a roubaix was that the 8r was much stiffer - but not on the tarmac. On the tarmac they were about the same stiffness, but the 8r was more comfortable to ride.

I highly doubt you'd be able to tell the difference in stiffness riding it, but the difference between the frames is more often in general ride characteristics than it is simply stiffness.