Well no kidding, if you shoot subsonic it's quieter.
Who the hell wants to shoot subsonic .223? You might as well shoot .22lr, it has almost as much energy.
And yes, a suppressed .45 upper will be a lot quieter than a "full power" (edit added for anal people) rifle round. In fact, all pistol ammo will be quieter than rifle ammo. Big suprise, eh?
Subsonic will always be quieter because your bullet isn't breaking the sound barrier. Pistol ammo will always be quieter than rifle ammo because there isn't as much velocity/bullet mass/etc. Not too hard to figure out.
For what a rifle round is, and for what the suppressors do, and for the *massive* differences in sound are .... you will be impressed by a rifle suppressor. When I can rock-and-roll in 556 without ears under a tin overhang all day and not need ears - that's a pretty damn good can, IMO.
Doc