Im aware CE is 50% phy 50% fire dmg, so is 50% of it increased by 100% vs amped mobs?

Hi

Basically yes, amp improve CE physical dmg part

Amp decrease enemy phy res by 100%, it does not double the dmg automaticaly, that depend on the base physical res of each enemy ...

If the're at exactly 0%, they'll got to -100% and that will double the phy part of CE dmg

If they are at 50% (which is pretty common in hell), they will get their phy res decreased to -50, therefore the phy dmg they take are *3, which is why you notice such a difference using CE with or without amp

Note that you can't lower under -100%

yes, and lower res/conviction would increase the fire damage