I wonder if this would have the same effect in terms of loudness protection for the shooter. The big thing argued with suppressors is that they just want them for hearing protection (guns ARE loud after all) -- so I wonder if this would appease some people. Loud guns for the people who want loud guns, and some hearing protection for people who want hearing protection :D (okay okay, I'm being a LITTLE facetious)
What you describe is known as a linear compensator and is fairly common. It doesn't reduce enough to protect the shooter's hearing, but it does direct the blast forward reducing concussive forces felt by the shooter and increasing the noise and concussive force to those in front of the muzzle.
these are pistol rounds and thus most likely subsonic. If it's a supersonic round then yes, but it is possible to hear the shot before you get hit. Possibly even more likely with this.
Likely not to an appreciable degree. Horns make a sound louder by giving the thing producing the sound more air to push against, so the pressure waves actually build up and travel through the air instead of just pushing it around.
Meanwhile a gunshot is loud because it's already a massive pressure wave. Silencers work by containing the pressure spike and allowing the gas to expand and cool.
In fact, muzzle cones were added to rifles and machine guns during the early 20th century for a similar reason, to suppress the muzzle flash from the unburnt powder hitting fresh air and combusting by giving the gases a place to expand and cool. They did also tend to direct some of the concussion away from the shooter, but it's debatable how effective this was.