I think there are characters with superhuman strength who still aren't as strong as Spider-Man. Dude can lift 10 tons over his head, after all. (I mean, he forgets he can do that depending on the writer, but still.)

I agree that mental attributes are less affected than physical, but they might still get smaller bonuses to represent the subject's sharper senses, greater confidence and ability to think faster.

The serum is a bit awkward to model because D&D's character progression charts people becoming superhuman over time. I mean, you can't tell me that a person able to kill bears and umber hulks with her bare hands or climb sheer stone walls without a rope doesn't have superhuman capabilities. So would a level 1 fighter treated with serum just become... uh... super-humaner as he levels up over time? Like, you accounted for the +5 to attributes from leveling - would our super soldier still get the attribute points as she levels?

It almost seems to me that "human/dwarf/whatever treated with the super soldier serum" could be a race with a level adjustment.