As written you could keep everything but in cases like Impervious, nothing says it is or should be limited to 20 which rightly limits damage 10 or below attacks. If you notice Batman can deal out 6 + 5 = 11 damage and in one recent movie I saw where he battles superman and other where he battles Ultraman (sups equivalent) and throws the batarang multiple times only to see no effect that is where most of us have the issue.
It is easy to fix that, simply raise the Impervious of superman. He has toughness 18 (and could have a 20 since his defense at 10 would still be in line with the concept but there is no reason he could not have a higher impervious than his toughness. What point do you want to deny damage saves? 15 damage? Impervious 30 would require a +15 damage to force a save (or you could choose Impervious 26. Knowing how Impervious and power attack and damage works that would actually create a mostly True Invulnerability Superman. Superman then has +20 toughness, Impervious 30 to match your conception of "superman"
Is Superman basically as invulnerable as Juggernaut? Something like that right.
If the number is set to 30 impervious then who can harm him? Any rightly strong person like superboy and above, but then it is slightly too much for anyone like Batman and his ilk, or Bane and similar level villains. At least with toughness damage. Fortitude and Dodge based attacks, and especially Will are then your best choices, just like the choices characters make in the comics. You attack superman with that "magical" weapon like Hawkgirl has and suddenly Superman is taking knocks left and right. Or the weapon or attack is 'will based' like Psychlockes attacks or something Dr. Strange or Fate would use.
The decision was made to keep a softcap of 20 for base stats but things like Powerlifting clearly go beyond 20 and since Impervious, isn't PL limited (or is there a stated limit for I'm missing that it is limited to your toughness, if so then by PL and stated limits; and since Superman is decided to be PL 15, then Impervious 20 is the most you can give him). But if Impervious stops attacks at half the rank, I see no imbalance or reason to limit Impervious to toughness. If you want to spend 30 pp to do that go for it. Yes, you would need to spend 30 pp x Toughness, Fortitude, and Will (90 pp) and you'd best be off just buying up Immunity which I think is the same cost.
How would you react if you had seen Superman with an Impervious over 20, say Impervious 30, with a Toughness of 20. Would that be sufficiently 'Superman' for you?
Keep in mind he'd have a Vulnerability complication to magical damage where he loses the Impervious ranks to those attacks and I don't seen why it needs to be where he also loses access to his toughness too since having a +20 is tough but we know full well he might face a +13 magical mace or something, or a +16 magical blast and he might have some toughness bonus otherwise a single hit might put him out, so I'd rather see like half of his 20, +10 vs magical attacks. He is harmed by them which is to say he is not Impervious but in all cases I've seen he does not fold like a house of cards, but merely reacts like "darn that really hurts" and reacts like a character who had to make a save and didn't make his save, and +10 vs say a +13 save means he rolls with a -3 and has a chance to do just that.
On the flip side he should get a hero point as its a complication, each magical attack he receives (or perhaps not? what do you think?)
The same could get applied to Juggernaut. He has and can be harmed, but has a recovery or magical regeneration so there is a threshold, but it is a threshold set very high and clearly one higher than a guy like colossus or wolverine can hit (and colossus might do like +12 to +14 with power attack for a total of +17 to +19 if you built him like a standard Power House so he could reach the Impervious 30 threshold). Perhaps the idea for Juggernaut should be Impervious 40 then only damage 20+ can force a save, but then there are characters that can do that like the Hulk, Superman, etc. and it is a bit too much for the likes of Thing, Colossus, etc. or Wolverine with those claws, though I can see his claws with a high range of Penetrating.
Back to Superman....