Well the Panther has 260-240-220 penetration. I don't see how that is not enough after the IS2 armour nerf.
Comet/AVRE/Croc has 290 armour so it will pen them at a very high % already.
KV1 has 270, KV2 has 300, ISU and IS2 have 340.
Pershing has 270 armour.
The Panther feels a bit lacking I agree but not because of penetration. It's more because it's somewhat difficult to use the mobility to full effect because you always need to micro every shot on the move by ordering stop shortly before you expect the shot to go off. If the Panther had more accuracy on the move it would be pretty good. This or slightly lower reload so it shoots a bit faster.
Obviously the Panther could also get 200 DPS with a reload nerf but I am not really sure if that is a good idea because it would make it worse against Allied 640 HP TDs. You will still need 4 shots to kill them but it will take you more time to do so. That's why I don't really like the 200 DPS suggestion. Also not sure if a Panther 4 shotting a Comet is such a good idea. It would be fine vs T34/85 but against Comet? Comet costs 175 fuel and 500+ MP. It would make Comets pretty much useless as soon as a Panther shows up. Comet already doesn't have the range, reload speed and penetration to fight Panthers as it is so the balance in that regard is fine. Panther has more AT capability and will 1v1 the Comet but the Comet is better vs infantry because of main-gun AOE, grenades and the phosphor round.
Penetration isn't the issue, firepower is. And Mirage's idea would fix that.
As for TDs, I don't think you ever want to get the Panther to deal with Allied TDs. Especially with the known issue of Panther moving accuracy.
As for the Comet, I mean the role of the Panther is to destroy premium mediums/heavy. And with the reload nerf, I think it would probably be an identical TTK to what we have atm. Only that it removes the frustration of using the Panther in it's current format.
The problem with the Panther is that there's virtually no reason to get it over Stugs, unless the map is severely unfriendly towards them (like Elst). You have this pseudo tank destroyer that has no better performance than Stugs. The only reason you'd ever get it is to counter super heavies, but those were nerfed recently so odds are, you won't need the Panther. As mentioned above, diving with the Panther is absolutely not an option due to its unreliability or you having to stop in the middle of enemy territory and taking the risk of being snared.
KVs, Crocs, Churchills, you're probably better off with multiple Stugs due to firepower.