The noise is caused by the high pressure gases escaping from the gun. While flash supressed powders do have a bit less energy than the non supressed powders, the difference isn't much. You might compare loading data between say AA#9 and 2400 or H110. What they do is cut down on muzzle flash, but, if they reduce noise much, I haven't noticed it.
The Speer Short Barrel ammo I've tried is pretty odviously loaded with a faster powder and not near as hot a load as CCI's full power ammo is in my 3 inch Smith.
So, from what I've seen, it is possible to have nearly full power, reduced flash ammo. E
