A couple of observations:
Haven't heard an extended recording of the PSE on the 718S so I can't speak as to whether it would be an improvement over stock. At the price they charge for it, IMHO, it would have to be a substantial improvement to warranty the $.
Regarding the Fabspeed exhaust system, it sounds more like a sports car exhaust than the stock version but still has the loud/odd overruns that the stock system has. My guess is that's part of the turbo setup that can't be changed.
From the video, the in-car sound--top up or down--didn't sound terrible or too loud, but you would have to listen to it in person to get a better feel. Having said that, for $2500 + installation, I think I'd pass.