I'm willing to contribute to this discussion.
Regarding lightning projectiles, I personally feel that the Vortex Pillar's lightning should only be considered as real lightning
due to being able to summon what appears to be cloud-to-ground lightning in the background and summoning lightning bolts to attack the players via generating them from various vortexes. Then again, that sounds iffy considering that there appear to be no clouds in the background and the lightning bolts are generated randomly in thin air, so this is definitely subject to change.
Otherwise, I think that speed should be scaled from the
Meteor Staff due to it summoning meteors that rain down from the sky. This could most likely range from ablative speeds at the very least (2,000-4,000 M/S) from a glance, to the atmospheric entry velocity for meteorites (11,000-72,000 M/S) if we take into account how the meteors in Terraria would work. If we factor in vortex Lightning as actually lightning and that it can move faster than these projectiles, then that could potentially make the MHS stuff consistent (If we're treating the meteors summoned from said staff as moving at 11-72 KM/S)
Likewise, relating to the currently speeds (MFTL+), I agree with removing it. It seems a bit outlandish to assume that the fallen stars and other identical projectiles are actually of that speed due to a statement regarding them (The starfury stars) coming from the heavens.
Anywho, if the 6-C feat is going to be purged at all,
I calculated the mass of the destroyer at one point (2,187,051.73766 kilograms) but didn't apply hollowness to it yet (Given that the destroyer is a robot), so I'll just do it now:
I'll do 90% hollowness (Multiply by 0.1), and 80% hollowness (Multiply by 0.2):
90% hollow
0.1 x 2,187,051.73766 =
218705.173766 kilograms, which is larger than the king slime's mass.
80% hollowness
0.2 x 2,187,051.73766 =
437410.348 kilograms, still larger than the king slime's mass.
Likewise, the mass is most likely a lot heaver given that it's constructed in such a way that it casually bores through materials including stone and metal without harming itself in the process. The kinetic energy calculation should not be accepted due to guessing on the behalf of the velocity as well as not factoring in hollowness.
For the Dwarf Star level tiering (Via moving the moon closer to the Earth), I don't see anything wrong with its statistic placement. Pulverizing a cubic kilometer of solid stone can be City level in spite of not being the size of a city. If that analogy was not good at all, rotating the Earth fast enough can be Large Planet level (and even Dwarf Star level if it's fast enough) at times. But I can see your point (That the moon appears to be in its original position while the transition happens), but another person can argue that it implies such. Otherwise, I don't really care that much if it gets reverted to 5-C, but I'm just pointing that out.
On the topic for the "At least" rating due to 'being able to move the moon at a fraction of their power', I can disagree with that easily. There isn't really much supporting that aside from it happening while the player fights against them. Saying that the Terrarian is superior to them is also pretty iffy if we consider how they can get heavily they can get harmed (and even killed) by the pillars and their attacks. I think they should only be rated "At least (Insert rating here)" due to being able to fight with the moon lord, who is superior to all of the towers (Which, as from my knowledge, is being used on the profile right now)