Lets assume best case scenario where you lock someone down and douse them with skywrath ult for 5 seconds, using a big ult (chrono, duel, black hole, roar, etc.) or stacking stuns. Skywrath aghs ult will have done twice the damage of a regular ult, over twice the duraction. With skywrath's old aghs, he could do double or triple the damage without stretching the duration. You would only need a single regular stun or hex to nail the enemy alchemist with 4529.7 damage after reductions (3 * 1400 * 1.0875 from silence amp) in 2.4 seconds. Plus, the longer duration of the new aghs will give enemies more time to react and help their ally with force staff or glimmer cape.
The only upside of the new aghs is that you dont burn through your mana pool with one kill.