In my limited experience, "SMG only" does increase the ROF in open bolt guns like the Uzi. The added pressure makes sure the bolt will travel far enough to engage the sear when the shooter lets off the trigger. As mentioned by James K, weaker ammo can cause run-away guns, especially if they are dirty.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer