How much does Cleansing Brillance actually do?

250 a second. Yeah, it's super strong. Keep in mind ships get damage mitigation though.

To be more precise, since the damage is not all dealt at once, the shield mitigation is recalculated for each tick of the ability. So, assuming nothing else is shooting the target, its mitigation will increase every second so that in the end the ability by itself does quite a lot less than its 2000 'on paper' damage, though still a very useful and high damage ability.

250 damage, for 8 seconds, for a total of 2,000 damage to any given target. Because it happens over time though, by the end of the attack most targets will max out their mitigation, so it's possible for a target to only take 1,000 damage, sometimes even less if they were in combat before the hit.

The real kicker is though that Cleansing Brilliance inflicts this same damage to any ships that get in the way of the beam. The beam also has a small area of effect outside the visible beam, so the ships do not have to be directly caught in the beam in order to take damage from it. So if the fleet is stationary and two or three additional ships are in the way of the beam, they will also take that same 250 damage over 8 seconds unless they move away.

So really to maximize the damage of Cleansing Brilliance, aiming for deep ship concentrations will serve you best. Team it up with a Progenitor Mothership using Malice and the crazy stuff begins, how to do that best is talked about pretty indepth over in this old thread Malice + Brilliance technical details.

I posted much too slow.

I can't give you hard numbers, but I know from having used it that it can best be described by its effects. In conjunction with Malice and intelligent choice of which ship to fire CB at, it does severe damage to nearby cruisers and absolutely rapes frigates. The last time I used it, an AI sent a capital ship and 56 disciples after my dozen or so Illuminators, Radiance, and Progenitor. I maliced the enemy formation and fired CB at the cap. Admittedly, it was a near ideal situation as the Disciple spam was in tight formation and the beam crossed right through the middle of them on its way to the enemy cap. But after CB had finished firing, it destroyed every last one of the enemy's disciples. No survivors, and his cap was almost out of shields.

Should this be so destructive?

It can be interrupted, and the Malice + Brilliance combo requires 2 ships to execute and can be interrupted with 1.

The AI isn't particularly great at interrupting it, but in Multiplayer it's hard to argue that it's unbalanced.

Yes... it was quite intentional, I'm sure. Damage isn't as difficult to balance as skills that do miscellaneous effects such as AOE stuns (subverter) or pushing your ships around (guardian)