Hunger for Power reduces Guldan’s self healing by approx 10% (after spell power healing buffs are applied). I tested it at lvl 16 with and without the Darkness within Spell power buff using Consume souls to try and pinpoint exactly what’s happening (had a 8.72% reduction of the total expected healing without the extra 25% spell power and a 11.65% reduction of the total expected healing with).
Similar numbers seem to apply to drain life but I tested it less extensively since the tick numbers smaller and less precise.