I am aware this idea may get shot down six ways to Sunday by other players. Still, I think it is a good suggestion.
You could make all morphs of Siphoning Strikes like the other toggles in the game that take away from a max magicka pool, but for this skill use health.
While the skill is toggled on, it reduces the player's max health by 10%.
Either of these would trade survivability for more damage instead of lowering the damage you are trying to gain. In addition, lower surviviablity would counter the sustain built into the skill as well.
Sort of like a Berserk or Kamikaze type of trade off.
Or something similar to that anyway... instead of lowering the damage we do, just make it so we are easier to kill if we turn on the endless resource regen.
Can do more sustained dps vs. Dies more easily seems like a good counter balance to me. Much better than the current option of can attack forever but at dps levels anyone else can beat.