I believe it's capped at 66% heal reduction. In other words, a 100 pt orison will always heal at least 33 points, even if ever single heal-reducing effect in the game is stacked on the target.
Deep Wound works independently of healing reduction.
A target that has a Deep Wound has his max health lowered by 20%. Apply max healing reduction on top of that, and he can only be healed for 33% of the healing spell's effectiveness, to a total of 80% of his original full health.