Generally I see hit points as a measure on how well you react to being physically damaged, how much pain someone can take. The more hitpoints you have the less effect 1 loss of a hitpoint will have on you. If someone has 10 hitpoints and takes 8 damage from a longsword the description would be something like "You cut deeply into the bandit's side, not quite knocking him down (i.e. into negs) but he is barely standing". Now if that same bandit had 50 hp and you did 8 damage it should sound like "Your blow connects to the bandit's chest, penetrating his armor, but he rolls with the attack and the force is only enough to cause a small cut, causing him considerable pain but nothing life threatening." People with more constitution can withstand more pain.
Against magic it works a bit different. 10 points of fire damage may be significant at level 2 but at level 12 its not. Now the character may be burnt the exact same way but the extra hp's means the character can withstand that pain and the body doesn't react the same way anymore. It may be a bit supernatural but it works.
Really I don't see a reason to tweak the system if you are just going to use large amounts of numbers someone has to burn through to kill/disable a character, essentially the same thing as hp.