As it stands, Latios needs 70% to switch in on Specs Fire Blast, and Latias needs 60%. Fire Blast also doesn't let them Roost for free because it is going to do the same damage the next turn. It doesn't even change any OHKOes or 2HKOes against them, regardless of entry hazards; I know that doesn't mean the two are perfectly equivalent, but we are talking about a 10% (of Lati@s' max HP) damage difference in a scenario where a pokemon with full offensive investment switches in against a bulky Choiced wall-breaker. That situation isn't going to get much better, and if Overheat is making it worse at all, it's still providing trade-offs at the same time.252+ SpA Choice Specs Analytic CAP18 Overheat vs. 4 HP / 0 SpD Latios: 215-253 (71.1 - 83.7%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Analytic CAP18 Overheat vs. 4 HP / 0 SpD Latios: 195-231 (64.5 - 76.4%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Choice Specs Analytic CAP18 Overheat vs. 4 HP / 0 SpD Latias: 186-219 (61.5 - 72.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Choice Specs Analytic CAP18 Overheat vs. 4 HP / 0 SpD Latias: 169-200 (55.9 - 66.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Yeah, they might be able to Roost off the damage if they carry it, but Latios shouldn't have to have 80% of its health in order to counter CAP18 and Latias shouldn't need 70%. I'm already concerned Fire Blast is too powerful, and Overheat just magnifies those concerns. Disallow Overheat.
On the other hand, Overheat guarantees a OHKO on Aegislash without a boosting item, while Fire Blast does not (assuming Modest). This is pretty major for any AV or Leftovers sets, and it's also very specific (to dealing with Aegislash). Leftovers sets can also use moves not affected by the stat drop.
To my knowledge, there are no Fire-types in any tier this side of DPPt that commonly run Overheat. I don't recall the last time I saw Overheat listed in Other Options on a smogon analysis of a Fire-type, either. So I have my doubts that you would even want to use Overheat on a Specs set. To hit Lati@s harder, but still not KO? You don't need it for Aegislash if you are running Specs, either. So what is the big appeal? There is a very specific reason to use it without a boosting item, but it's probably sub par outside of that.
Psychic and Psyshock: I just want to point out that Keldeo is on the list of pokemon that threaten us. It's listed as "to a lesser extent", meaning it shouldn't counter us, but if we can OHKO it then suddenly we check it rather than the other way around, and that's not what we want either. Lets leave it as a mutual 2HKO, where it has a slight edge with better speed. Apparently Psyshock is good against Goodra, too. I don't see any redeeming qualities to counteract these flaws.
Disallow Psychic and Psyshock