Well I guess all we can do is throw around opinions. Unless we get the guns on the range together to compare. Without proof, it's all hearsay.
However, I have seen guns that shoot close to 425 FPS (Our league limit in Humboldt) and they are so quiet you would have to hear it to believe it. One weapon comes to mind that got us all chuckling in disbelief it was so quiet. And It was right at 425 FPS.
It's not just air that makes noise in an AEG. Most of the noise comes from the motor and the gearbox. A weaker spring will increase ROF sometimes. But a higher voltage will incease ROF everytime. My sopmod shoots 26.5 Rounds per second at 420 FPS and it is more quiet than all of my other guns. And Some of them are below 400 FPS.
FPS is directly related to impact energy and range. Not so much with accuracy. There is much more of a chance of the target feeling and calling a hit with a higher FPS BB than with a low FPS BB.