The first time I heard a suppressed M16, I was very impressed. This was around 10 years ago, and with an AWC can on a relatively short barrel. To me, it sounded like a .22 rifle.
However, my perspective changed a little when actually shooting a suppressed AR15.
One thing to consider it that an AR15, while not excessively "loud", is still not the quietest action out there either. With an AR15, you feed gas back into the bolt carrier to work the action. Once the bolt unlocks, the gas escapes out the vent holes in the side of the carrier at the ejection port. This is relatively close to the shooter's face, so the shooter will get some extra (unsuppressed) sound from the ejection port area.
While a gas piston gun would not have this issue, it dumps gas at the gas block. This moves the gas release point a little farther away from the shooter (and closer to where you typically take sound readings). I don't know if the perceived sound levels would be louder or quieter on a piston gun.
But with a direct-impingement gun, the shooter will get some extra noise close up. Some people have made the claim that there are no hearing-safe suppressors for an AR15 if readings are taken at the location of the shooter's ear. (I don't know this one way or the other, but neither of my .223 cans are entirely "comfortable" to shoot with on a 10.5" or a 16" barrel).
If comparing the effectiveness of a particular can, it might be better to try them on a bolt-action and take the mechanical and ejection port noise out of the equation.