the 400fps rating is the factory raiting for the M120 spring set up. All SRC Gen 3 guns come with M120 spring, but as you know, the FPS is also dependent on air compression, seals, hop up setting, barrel bore, and most importantly, Barrel length.
So yes, the actual FPS of each SRC Gen 3 gun will vary. Here are a few that I have chronoed, although they have the same guts and system inside of the their gear boxes:
M4 Micro SD ~320fps
M4 CQB ~360 fps
M4A1 ~405 fps
MG36 ~415 fps