So in the handful battles I've had, I've found quite a few things out about both Cofagrigus and Aggron.
A poor man's Rhyperior
Aggron | Life Orb
Rock Head | Adamant Nature
4 HP | 252 Atk | 252 Spe
Automotize | Head Smash | Earthquake | Aqua Tail
-Automotize set isn't good for very much but surprise KOs. Sure, after one boost, you're hitting ~400 speed, but it really does nothing to help Aggron but gain a few surprise KOs, as mentioned.
-The number of bulky Water-type and generally defensive Pokemon in UU really hurt Aggron. Head Smash from the Automotize set only does about 61% to Suicune (Adamant / 252 Atk / Life Orb), and laudable damage to Slowbro, Swampert, and Milotic, all of which dick it over with Scald. Gligar only takes about 45% with the same investments on Aggron, who then proceeds to fuck it over with an Earthquake.
-Aggron doesn't like taking any damage unless its a x4 resisted physical attack. Even neutral hits sometimes hurt this thing. Special Attacks easily do it in.
Despite being shut down by a large portion of the tier, Aggron does fairly well once the checks are weakened beyond 60% or completely removed. I've ran a Stun Spore Roserade to compensate for the low speed it has before setup so it doesn't rely on Automotize if needbe, and not much outside of the checks likes taking a STAB Head Smash, and having the attacker take no recoil. Defensively, as long as its a physical hit, neutral or lower, Aggron takes it with no problem. With no investment, it carries 396 defense, the same as standard Cofagrigus, just with better physically-defensive typing. Aggron appreciates an increased attack stat, and either Paralysis support or Trick Room support, much like it does in RU. A base 350 attack stat just doesn't hit very hard these days, even with a base 150 STAB move. Rhyperior just does what Aggron does better, and it gets a better ability to keep it alive longer.
WHY U NO UU?!?
Cofagrigus | Leftovers
Mummy | Bold Nature
252 HP | 136 Def | 120 SDef
Shadow Ball | Will-O-Wisp | Rest | Haze
-Absolute best defensive check to such a large number of physical threats in the tier, and an amazing spinblocker
-Shuts down the top 5 physical attackers in the tier
-Doesn't like taking powerful Special attacks, but can take a few (defensive Roserade's Giga Drain does ~25%, Blastoise's Scald only does ~30%)
-Suffers from FMS
Most likely the highlight of my research experience so far. Cofagrigus can generally take abuse from physical threats all day. I've just been using a standard physically defensive set, and I've rather enjoyed the experience. The top 5 physical attacking threats in the tier can't really do anything to Cofagrigus. The combo of Will-O-Wisp and Mummy destroy Heracross and other Guts abusers, which is dead useful. Access to Haze is a major boon too, as it makes Curselax such a lesser threat than it could be, and it turns Hitmontop into dead weight. The FMS really comes into play against Normal or Steel-type physical attackers who would otherwise take jack shit from Shadow Ball (I run Shadow Ball / Rest / Will-O-Wisp / Haze, Rest ensures longevity and dicks over Roserade and Sleep Clause), and could make such good use of Focus Blast (if it gets it) or Hidden Power Fighting. If this thing does come up to UU, everyone should be using it for spinblocking. It just supports teams so well. And it looks damn cool.
I've tested out some more offensive sets, Choice Band for Aggron and Trick Room for Cofagrigus, and here's what I've found out:
It's best set
Aggron | Choice Band
Rock Head | Brave Nature
252 HP | 252 Atk | 4 SpDef
Head Smash | Heavy Slam | Earthquake | Aqua Tail
-Hits extremely hard; easily 2HKOs Gligar
-Does work under Trick Room, extremely predictable however (spam Head Smash)
-Breaks stall in half
Drastically different results than from the previous Automotize set. aggron really isn't cut out for the set-up sweeper business, and would rather just drop shit out of the sky whilst holding a Choice Band. It can OHKO over half the tier, and most everything else is 2HKO'd (actually, there's only 7 Pokemon in the UU tier that aren't 2HKO'd: Registeel [~35%], Rhyperior [~45%], Swampert [~35%], Hitmontop [Factoring Intimidate ~30%, without ~45%], Cobalion [~25%], Hippopotas [~25%], Bronzong [~35%]). If using Aggron, this should be the set everybody uses, as it's quite dangerous, especially with hazards support and Paralysis or Trick Room support.
Where's your Pink Blob now metgame?
Cofagrigus | Leftovers
Mummy | Quiet Nature
252 HP | 4 Def | 252 SpAtk
Trick Room | Nasty Plot | Shadow Ball | Hidden Power Fighting
-Trick Room turns it into a massive threat
-Can set up on special walls and beat them
-Everything in the UU tier barring Snorlax and Bronzong are either 2HKO'd or OHKO'd at +2 by either Shadow Ball or Hidden Power Fighting
-Max HP / Max SpDef Snorlax is 2HKO'd by HPF at +4
-Still difficult / borderline impossible to break via Physical attacks
And I thought Cofagrigus didn't get much better. I've always looked down at Trick Room as being a shaky playstyle. However, Cofagrigus gives it a lot of validity. The Ghost-Fighting attacks provide excellent coverage which does, as mention, really decimate the UU tier given the chance to set up. for shits and giggles, I ran a calculation with a +6 SpAtk Cofagrigus and a Wish set Chansey, and there's a strong chance for a 2HKO (48.86% - 57.67% / 344HP - 406HP). Not many special setup sweepers can hit Chansey that hard, even with that many boosts. Cofagrigus is very strong, both offensively and defensively, and should become an UU staple when and/or if it moves up from RU.
Potentially more to come.