From what i remember their powers were probably based on a belief system like spirits or you tube channels,more followers more power.
In modern Thedas if Solas will release them they shouldn't be technically as powerful as they were in Arlathan.
I don't believe that they derived their power directly from belief, though with Thedean magic it's already kind of weird. It seems that magic there requires manipulating reality by force of will, so perhaps each Evanuris is a particularly strong-willed mage, and when exerting that will upon the magical world they lived in (including the elves in it) it more naturally bent itself to their desires.
As far as if/when they are released, whether they will be as powerful, it kind of depends on what exactly Solas did to them to be able to entrap them in the first place.
Did he weaken them? Were they already compromised by the taint/the Blight? Did he simply trick them into going into a room behind an Eluvian they could not break out of, where they've each been building up power for thousands of years? Are they the ones "asleep, masked in a mirror" that Cole refers to? Perhaps Solas tricked them into putting on some sort of device (mask) that would nullify their powers temporarily, and then quickly locked them behind an eluvian before they could figure it out. I personally believe that this last theory is very likely, and that this mural in Trespasser is Solas' (very self-congratulatory) commemoration of his deed, and each of the seven large panels, represents a trapped Evanuris, faceless, powerless, behind a dark mask.
Since we don't know the specifics, but Solas seems concerned about them, and Mythal/Flemeth believes she can still get her heaven-shaking revenge on them, and Abelas in the ToM says they must prepare for the ones who cast Mythal down, I believe they are still a credible threat, with abilities on par with those Mythal/Flemeth and Trespasser!Solas have demonstrated.
How do you know that though? What evidence was there that the entire Veil's intergrity was failing? Kirkwall was certainly a soft spot in the Veil, but what about the rest?
It may or may not have been failing before the Breach, but once a big enough hole is torn in it, it does begin to disintegrate, or shatter in a very horrific manner, as seen in the future world that you jump to in "In Hushed Whispers".
We do not know what method of the Veil's removal is worse, whether if letting it disintegrate naturally would be worse than, say if it is ripped off quickly like a band-aid as Solas wants to do. But it is known, because of "In Hushed Whispers", that if it is left damaged enough and allowed to degrade, it can completely fail. So, if there is, as you say, a "soft spot", and it is left unchecked, it may eventually become like a worn out patch in the knees of a pair of pants, threadbare, or non-existent, and the less integrity there is to the whole, the more likely it is for the whole thing to fail and break down completely.
And to add a little tinfoil, I am personally of the belief/opinion, that to remove the Veil in the manner he wants to, there is something in the Fade that Solas has to get to, perhaps some sort of device in the Black City, which generates the vibration that maintains the construct that people in Thedas call the Veil, as well as the Evanuris' little eluvian pocket prisons. Mostly because I don't believe something that big and that world-altering, could maintain itself for that long without a power source. Why he hasn't just gone and shut it down already, since he can ostensibly enter the Fade at will like Flemeth/Mythal could (as shown by her dragging Morrigan's son into the Fade via eluvian, in the worldstates that allow for it) probably has to do with his orb being part of the mechanism, so he has to find another orb or another means. Anyway, tinfoil over.