just to nitpick you forgot filter on mega aggron, ik from experience talonflame needs +2 to 2hko aggron without a high role tho thats without a LOHeatran isn't going to be included, given neither Flare Blitz nor Overheat affects it at all.
Let's assume a Life Orb for a fair chance, and without an SD boost, since Choice Band or being at +2 guarantees Flare Blitz will outdamage.
252+ Atk Life Orb Talonflame Flare Blitz vs. 252 HP / 0 Def Scizor: 816-967 (237.2 - 281.1%) -- guaranteed OHKO
...Given Flare Blitz overkills the standard Scizor so hard, it's not even worth calculating the alternative. That's enough to 1HKO even if rain or reflect is up.
252+ Atk Life Orb Talonflame Flare Blitz vs. 252 HP / 48+ Def Ferrothorn: 567-671 (161 - 190.6%) -- guaranteed OHKO
Still a 1HKO on standard Ferrothorn.
252+ Atk Life Orb Talonflame Flare Blitz vs. 252 HP / 0 Def Aegislash-Shield: 289-343 (89.1 - 105.8%) -- 31.3% chance to OHKO
0 SpA Life Orb Talonflame Overheat vs. 252 HP / 0 SpD Aegislash-Shield: 198-237 (61.1 - 73.1%) -- guaranteed 2HKO
Speaks for itself. Univested Overheat can't 1HKO Shield Forme, but Flare Blitz can. Not even gonna bother with Sword Forme since I think we all know it'll be 1HKO'd in that form regardless.
Mega Aggron is the exception.
252+ Atk Life Orb Talonflame Flare Blitz vs. 252 HP / 4 Def Mega Aggron: 198-234 (57.5 - 68%) -- guaranteed 2HKO
0 SpA Life Orb Talonflame Overheat vs. 252 HP / 252+ SpD Mega Aggron: 237-281 (68.8 - 81.6%) -- guaranteed 2HKO
In this case Overheat does outdamage, but it's still a 2HKO either way.
The question becomes whether or not it's worth it to carry the move specifically to outdamage Mega Aggron when it's still a 2HKO either way. And the answer to that question would be no, it's not really worth it when there's much more viable moves to carry. Roost, Swords Dance, U-Turn, even Will-o-Wisp would be better.