Hi Voltron,
If your main goal is to prevent the outside noise from entering your listening space, the best (really, only..) solution is more mass.
Easiest solution, but far from only. Widening the wall helps keep low frequencies from going through. Adding a furring or changing the wall to something decoupled would have a very big effect.
You could even do better than just adding a layer of gyp without changing the structure at all. You can add a layer of gyp over green glue or the like. Or, instead of a layer of plain gyp, you could add a layer of QuietRock (or similar) for a pretty big improvement.
The easiest and cheapest way to do this is by doubling the drywall, just glue another sheet of drywall directly to the existing. Stagger the joints so the "cracks" between the drywall sheets don't line up.
It is the easiest and the cheapest, but I don't know if it'd be a lot more to add green glue.
You can't really "reflect" outside noise away - but mass will stop it.
That's actually what happens to most of the sound that doesn't get through your wall, it bounces away. Higher mass means bigger impedance change between air and wall, so there's a less efficient energy transfer between the two mediums. What doesn't get transferred to the wall gets bounced away. This happens in all mediums, not just air. Most of the vibration traveling through a board will bounce back when it reaches the end of the board for the same reason, because there's a big impedance change where the board meets air. Some sound is absorbed as heat into the wall panels, and some of it is trapped in insulation (becoming heat).
Without invasive demolition, I am expecting standard 2X4 framing with 1/2 inch drywall on both surfaces.
That would be pretty flimsy for an exterior partition. There's at least going to be insulation, and I don't think you can get type X in any size other than 5/8. Type X is the stuff with fiberglass infused and it's required for walls with a fire rating, as the fiberglass holds the gypsum together after the paper burns off. Anyway, you'd expect to find 5/8 on at least one side, and usually exterior studs are bigger than 2x4.
Putting perforated board over the insulation will basically mean it won't help reduce sound transmission through the wall, but it will help with absorption because it lets the sound through to the insulation where it gets sucked up.
Non-perforated will mean it's acting like an insulated furring. That will increase transmission loss, but it won't do anything for absorption, except at low frequencies where the board will act like a panel resonator.