Couple nitpicks. For one, that first calc is against a Mega Slowbro, so it should look like this:Alright, I want to hang the disclaimer that the mon I'm nomming is one I really love, so there MAY be a biased to consider.
Tyrantrum to C+
Now, I love Tyrantrum for his Choice Band set. Head Smash can basically barrel through any core without a decently bulky resist with some help from SR.
252+ Atk Choice Band Tyrantrum Head Smash vs. 252 HP / 232+ Def Slowbro: 181-214 (45.9 - 54.3%) -- 96.9% chance to 2HKO after Stealth Rock and Leftovers recovery
That said, his speed, mediocre Special Bulk, and lackluster defensive typing make him extremely susceptible to revenge kills, his speed barely above Bisharp's and leaving him outpaced by a lot of stuff even on Balance teams. Choice Scarf sacrifices significant power, and Dragon Dance sets have a very hard time trying to set up.
Now, these were all issues before, but one thing that has changed I think compounds these issues: Hoopa's entrance into the meta game.
Hoopa gives Tyrantrum heavy competition for the Mega Slot, possessing a better (even if not necessarily good) defensive typing, a better Speed tier (outpacing Heatran and being able to tie with Base 80's is certainly valuable), phenomenal coverage, and mixed potential in exchange for going from ludicrous power to just extreme power. Now, Tyrantrum certainly had competition as a Wallbreaker even before, but Hoopa's presence coincides with/accelerated a metagame shift to more offensive teams, whether teams becoming full out offense or just leaning more towards it in balance. Tyrantrum does not fare well against offensive teams for the reasons I noted above, so until/unless the metagame shifts defensively again (unlikely in the near future with Zard-X, Manaphy, and Hoopa-U occupying the upper ranks), I feel he's more in line with wallbreakers like Staraptor, whose absurd power and offensive option can do some teams good, but usually is going to need support more than he will support anything in his role, compared to Toxicroak, who can soft check a lot of Water/Rain mons and has Sucker Punch alongside STABs to help check/revenge things like Gengar, the Latis, Keldeo, Azumarill, and LO/Mega Alakazam on offense, while also being good against typical members of a Steel-Fairy-Dragon Core such as Clefable, Altaria, Heatran, and some others depending on what he carries for a 4th Move.
In retrospect, I'm also a fan of what Toxicroak can offer a team, but I think Tyrantrum is more likely to drop than he is to rise.
Tyrantrum to C+, Toxicroak to B (?)
Even if neither of these take, they're two somewhat lower ranked mons I think could spark some discussion.
252+ Atk Choice Band Tyrantrum Head Smash vs. 252 HP / 232+ Def Mega Slowbro: 181-214 (45.9 - 54.3%) -- guaranteed 2HKO after Stealth Rock
Against regular Slowbro, it's more like:
252+ Atk Choice Band Tyrantrum Head Smash vs. 252 HP / 232+ Def Slowbro: 262-310 (66.4 - 78.6%) -- guaranteed 2HKO after Leftovers recovery
It's a small thing, but you seem to be underselling it a bit -- Band Tyrantrum's Head Smash is, last time I checked, the fourth strongest move in the tier, after Mega Glalie's Refrigerate Explosion, Mega Medicham's High Jump Kick and Band Victini's V-Create, all of which you'll note have huge opportunity costs. With Tyrantrum, though, he has none bar Head Smash's shaky accuracy -- get him in, which isn't as hard as you're making it out to be, and something dies. I'm serious: Tyrantrum's Head Smash gives you three options. If it's Rock-resistant and physically bulky, it gets 3HKO'd. If it's either of those, 2HKO'd. None, and it dies -- seriously, no exceptions. Additionally, its physical bulk is quite good -- you'll never invest in it, but it can allow it to live some surprise hits in a 1v1 or to finish off a 2HKO, which no-one expects because no-one uses it. It can live stuff like CM Keldeo's Secret Sword, Scarf Excadrill's Iron Head, and SD Mega Scizor's Bullet Punch and murder them in return. And to top it off, a ton of things that can live a Head Smash get destroyed by Banded Outrage, which is a risky move but when the payoff is a OHKO, hey, I'm not complaining.
Now, I'm not trying to make it out to be some god of a Pokemon. The flaws you listed (middling speed that only lets it surpass defensive mons, dies to a sneeze as long as it's special, mediocre defensive typing) are quite valid. However, I think that they by no means surpass its virtues and so Tyrantrum does not deserve a drop. You want a cost-free nuke, look no further.
(Also, I believe you meant competition for a wallbreaker? Hoopa-U is probably the best wallbreaker in the tier right now, true, but they don't fill the same niches and it shouldn't affect Tyrantrum's placement in the same vein that Kyurem-B shouldn't drop, either.)