The differences in two different bores, chambers and throats will make more velocity difference then 2" of barrel length in the 7mm Rem Mag.
IF all things were the same, you would likely see less then 20 fps gain per inch. Its not nearly as much as most think it is.
I have built some 22" 300 WSM
rifles that were faster then some factory 26" 300 WSM rifles. Its all in the chamber, throat and bore diameter for speed.