Well draining HP from the mob implies you are taking their HP to restore your own. 10% of 50,000 hp would be 5000. Draining 5000 HP from the mob to give you 5000 is still a DPS increase cause you are doing 5000 damage to them.

If Soul survivior became a flat 10% HP gain and then 10% after its killed without any kind of drain there would be no DPS increase and it would work fine.