I think I have some plausible solutions.
A few things:
-As many have stated, the more ultras you have, the more useful and viable they are.
-In order to fix Ultras, I suggest 1 of 2 things:
1) Fix the damage "ratio" and make it consistent. Bring their health back up to 600. If you're going to make their normal attack weaker (15 against non-armored), then atleast give them health so that they can do the same amount of damage pre-patch 13. What I mean by that is:
Pre-Patch 13: Ultra Attack = 25
25*Time = X Damage
Post-Patch 13: Ultra Attack = 15 (40 vs. armored)
15*Time = Y Damage
Try and make X and Y be as close as possible. In order to do so, you can increase the Ultra's health and/or make it's attacking speed faster.
2) Bring old Ultra damage back and keep the new Ultra damage.
Make the Ultra a viable unit. For the amount of resources it costs, it should be worth it.
Therefore, I suggest making it do 25 (40 vs armored). This way, Ultras aren't only used against Armored composition Units.
I think the Ultra will soon become a unit that is feared and utilized but we'll have to wait for Blizzard to get them to that point.