Benefit of shielded ac wire? Any downside?

In-wall or power cord? What are you talking about? Of course there is a downside.

I have made my own shielded power cords for decades using three conductor 12 gauge cable with a foil shield. I hook the shield up on the house side only. Does it help? I have an extremely quiet system but I can not tell how much the cords add to it. There is certainly no down side as long as you do not hook up both sides of the shield which might produce ground loop hum. There are lots of very sturdy plugs out there. I use Marinco plugs.

Sharper sounding but actually dirtier transients, combined with a subtle mushing of the dynamics.

Basically the same thing it does to all audio cables, but on the AC.

Immunity/separation/isolation from some types of noise.... with a side order of signal damage.

teo_audio said it all tec like, but that is the just of it. Different combinations of shielding can KILL the dynamics of an amp because of a "power cable". Poorly designed power supply doesn't help either along with a incorrectly shielded power cable and then use an AC filter and maintainer.  Disaster, SQ wise, if your not careful.

Regards