Belly Drum: the only reason this is more dangerous is extremespeed. Get rid of espeed and lopunny is the only pokemon that gains bdrum that would consider keeping it.
+6 252+ Atk Pixilate Mega Altaria Quick Attack vs. 0 HP / 4 Def Thundurus: 441-520 (147.4 - 173.9%) -- guaranteed OHKO
Quick Attack does, of course, fail to push past other priority, but that just brings us to "is a 25% cut in firepower worth pushing past enemy priority?" because Feint is also a Normal type move.
You don't need Extreme Speed for Belly Drum to open up priority sweeps. It helps, obviously, being twice as hard hitting and able to push past most other priority moves effortlessly, but Quick Attack Belly Drum can still be used to completely destroy offensive teams given one turn to setup. It should come as no surprise that this OHKOs Sableye by a large margin, so it's not like Topsy-Turvy is going to save you. Even if, for some ungodly reason, Trick Room is up so Sableye gets the advantage, we again return to Feint.
This can be replaced with the likes of Kangaskhan's Scrappy attack to achieve the same basic idea even assuming a Mega Altaria ban.
Surrounding Judgment, that's a fair point, yeah. I'm yet to see a Dark Void Weavile and, for that matter, a Weavile in general. I wouldn't ban the move; I'd lean towards a Sableye ban simply because of how good it is at shutting down set-up sweepers, gaining momentum and putting mons to sleep using Topsy-Turvy, Parting Shot and Dark Void, respectively.
Seen Dark Void Weavile, seen Weavile in general, loathe the damn thing. It can completely destroy Physical walls that expect to stall it indefinitely, and it has native access to Swords Dance. All it
needs is Dark STAB+Ice STAB to push through most of the meta, too, so the fact that Dark Void+Swords Dance limits it to two attacks doesn't doom the build. It's only real flaw is being afraid of priority.
I think Shell Smash deserves a ban too, but not Belly Drum. Belly Drum is really hard to set up versus offense, +6 Extreme Speed is stopped by Mega Metagross, and it is useless against Quagsire / Clefable. Belly Drum is good, but not overwhelming or centralizing to the point of being banned. I'm not in favor of banning Boomburst if Shell Smash is; but I'd rather both be banned it doesn't matter. I feel that Boomburst + Shell Smash is the problem, but not Boomburst itself. Can we start with just Shift Gear + Shell Smash banned and I guess Belly Drum if other people want, and then see how good Boomburst is?
The problem is that Offense
dies if it slips up and lets a Belly Drum get going. Stall has a lot more tools for blunting/stopping a Belly Drum sweep. Offense is leaning almost entirely on the offensive pressure preventing a Belly Drum from happening.
I do agree that banning Boomburst seems a bit premature, given how few Pokemon
actually run it and how one of them -Mega Altaria- is liable to be broken regardless of the exact banlist.
All Hail Eevee General. Of the moves on the radar, Dark Void is the only one that worries me. The other two are completely mitigated by a Sableye Topsy-Turvy (centralising, much?)
Anything that
has Geomancy is something Sableye can't safely switch in for fear of exploding to a Moon Blast.
Shift Gear is more debateable.
(P-Z has Adaptability, which basically gives it something akin to Tinted Lens).
I largely agree with your post, but a 33% boost to damage is
not comparable to a 100% boost to damage.