I'm still undecided personally, but let's refocus on the part that makes Torchic potentially broken rather than its middling offensive output IMO
I've been mulling over Torchic for the last couple of days and I think I've come to a conclusion for myself. Torchic is broken because it provides overwhelming team support with the following qualities:
a) Passing boosts is not stoppable unless you are running one of a handful of specific phazing sets (Haze Mareanie, Roar Onix, etc).
b) Torchic is extremely consistent thanks to a powerful offensive set, passive boosting, and overall lack of matchup difficulties.
c) It is impossible to prepare for the variety of good receivers in the tier, especially since speed tiers are so incredibly important in LC.
These factors in combination with one another create a threat that is virtually uncheckable, except with a handful of specific sets, and one that limits the possibilities for teambuilding to an extreme degree. This list does not include various other factors that make Torchic good, such as having the option to always outspeed unboosted Pokemon (Protect; can also outspeed Scarfers that come in on the switch thanks to Protect fail -> Protect), tanking most priority comfortably enough to pass or attack, and hazard clearing being at an all-time high.
To add to each thought:
a) Passing boosts is not stoppable unless you are running one of a handful of specific phazing sets (Haze Mareanie, Roar Onix, etc).
Pretty obvious. You cannot prevent Torchic from passing unless you run those, or unless you make a prediction and get a faster Scarfer in on the Protect (or Corphish). It's hard to dismiss or argue "counterplay" in a suspect discussion, but in a average reasonable game with two players who know what they're doing, it's fair to say that such plays are difficult to make and require a lot of risk, especially when Torchic gets
completely free turns against Pokemon like Ferroseed or Foongus.
b) Torchic is extremely consistent thanks to a powerful offensive set, passive boosting, and overall lack of matchup difficulties.
I will mostly be talking about the Firium set because it's the best IMO. Torchic gets opportunities to KO basically anything that is sub 14 Speed and is either weak to Fire or knocked/chipped. This is super detrimental to Torchic's opponent, because it removes the need to make plays. Torchic can just click Z-Fire Blast (btw why don't we run Z-Overheat? confirms stuff like OHKO on unbulky timb, has much better rolls vs basically everything, and not often are you using Torchic to clean) and get a risk-free turn with passive boosting.
Overheat: 200+ SpA Torchic Inferno Overdrive (195 BP) vs. 116 HP / 76 SpD Eviolite Vullaby: 16-21 (64 - 84%) -- 50% chance to OHKO after Stealth Rock
Possible damage amounts: (16, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 21)
Fire Blast: 200+ SpA Torchic Inferno Overdrive (185 BP) vs. 116 HP / 76 SpD Eviolite Vullaby: 16-19 (64 - 76%) -- 6.3% chance to OHKO after Stealth Rock
Possible damage amounts: (16, 16, 16, 16, 16, 16, 16, 16, 18, 18, 18, 18, 18, 18, 18, 19)
Additionally, Torchic does not typically face a bad matchup. You literally cannot have a bad matchup with Torchic unless you're playing against mono(resist) or Onix/Mareanie or Slowpoke/Staryu/x/x/x and even those are workable because you need a Snivy answer and Torchic can boost on basically all of those.
c) It is impossible to prepare for the variety of good receivers in the tier, especially since speed tiers are so incredibly important in LC.
This is the critical part of the argument. You cannot prepare for Torchic because you cannot prepare for all possible receivers, and it's crazy difficult to check things like (edit) Mudbray at +1 Speed. Even with options available like Haze Mareanie, Torchic always nets its user momentum. Passing into a Snivy means Snivy can knock Foongus for itself, Mienfoo, and Mudbray later in the game. Passing to Mudbray means its going to 2HKO *something* (reminder: Mudbray 2HKOs everything), etc.
Plus, there's flexibility in what can receive boosts thanks to the vast majority of LC-viable Pokemon being either reasonably bulky or strong, or often both. I built a Torchic team with Krabby as a receiver and it worked fine.
The fact is, one Pokemon cannot check everything, and Torchic can always put its player into a favorable situation (often not just a favorable matchup, but one with a boost on a PKMN that can't be easily revenged) thanks to BP.
Torchic's constant advantage state forces players to build teams to deal with it passively (unless you run Corphish or Tirtouga or something), which leads to the influx of teams with the same structure we've been seeing on ladder. Even teams with a reasonable matchup against Torchic must rely on handling it passively, by taking its hits and trying to KO the receiver in return. I'm not even going to touch the Torchic-answers-are-weak-to-Diglett meta discussion, which has already been brought up plenty and is also true.
P.S. This is a good calc: 132 Atk Tirtouga Aqua Jet vs. 0 HP / 36 Def Torchic: 14-18 (66.6 - 85.7%) -- 6.3% chance to OHKO after Stealth Rock
Possible damage amounts: (14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 18)
In summary: I don't think "Torchic can be beaten with Haze Marinara and friends" is a good enough argument. These Pokemon can become a liability with Baton Pass, and they cannot handle each possible receiver that Torchic can pass to. Torchic is head-and-shoulders above the rest of the competition in terms of consistently generating advantage, and the boosts make its receivers nearly impossible to handle offensively. These in combination put too much of a limitation on teambuilding, leading to a passive metagame that centers itself on handling Torchic. I believe the bird is broken.