For horror, reduce the HD to the next lowest. D6 =>D4, max DH to 9. There after +1 hp per HD type above D4 (D4 included). Monsters stay the same, remove the healing over night, all healing spells add in the optional rule of horror factor and voilà! It will work.
Does not do super? Well, a .12 gauge does 2d8. The average 10 level fighter has about 94 HP with 16 CN. This means that the fighter will take 10 to 11 shot gun shot before falling (on average). If this is not super... nothing is. Hey! A dynamite stick does 1d6 damage per stick with a maximum of 10. Your wizard can throw a fire ball at level 5 that does the equivalent of 8 sticks... and his fireball is still better than a hand grenade (5d6). And the most wonderful thing? A simple night sleep will remove all that damage you suffered. It might not do superman, but Batman can be done easily, Dr Strange can be done, heck, Cpt America is easy to do too! And in a super hero game, who wants to play the invulnerable character? It gets boring quite fast. No, you want to do the Iron Man (artificer), The Dr Strange (wizard, sorcerer), Captain America (Fighter, Ranger, Paladin) and whatever. Hey Spidey can be done too (arcane trickster, change the spells and make them transmutation and alteration, spider climb, web and so on.) If you get yourself to it, you will make it work.