100% Penetration against everything except KV"X", ISU152, IS2 which are all doctrinal heavy tanks. Still, Panther wins against every KV# tank and wins vs ISU152 and IS2.
Just a correction, the IS2 usually comes out on top 1v1 with a panther.
Also in my experience stugs do not usually "force away" pershings, the equivalent would be an M10 forcing away a panther.
I disagree with OP wanting a buff to panther RoF since I think it's fine.
But honestly stugs specifically are in a weird position imo. I find that you really only ever get them if you are far behind your opponent in armor, sort of like the Su76.
They aren't underpowered persay but they fill a weird niche between the panzer 4 and panther without anything positive that they exclusively can do unlike the JP4(60 range).