Only reason to change fans on the radiator is for appearance or aesthetics purposes. While sp does play a role somewhat in radiator cooling, that's only at lower cfm/rpm. The stock fans are purposed for the radiator, if they didn't perform well at all, Corsair wouldn't spend the cash to put them in the box as nobody would by an aio that really didn't do anything.
CFM is a standard, doesn't really matter how you test (much) the results are the same, cubic feet per minute. SP is different. It's the amount of liquid moved by the fan pressure. That makes a huge difference by testing methods. Blowing liquid downhill, across Teflon will result in far higher sp than uphill across sandpaper. So results are variable depending on method tested. So sp is not really a quantifiable standard as such and the measurements for one fan could be vastly different if tested by someone else. Meaning SP isn't something to rely on when looking at fans for changing out. An example would be the Noctua NF-F12, one of the best rad fans produced, yet has lower cfm, lower sp than a scythe GT, which isn't its equal.
So if you plan on changing fans on the rad, just replace them with fans of equitable known performance because statistics like sp aren't going to be reliable sources of performance info.