You would reduce by 800 still. It is a set value, unaffected by if the damage is reduced or outright nullified by another card effect. Consider it more a chain of effects, rather than one effect begetting the second effect.
The question is about if the "inflict 800 damage" condition is being fulfilled, not about the stat reduction. Under Damage Diet, you inflict 400, not 800, so technically you are not fulfilling the part A of the effect to proceed with part B. Think of how Bottomless Trap Hole won't banish the monster if it didn't destroy it.