Usually when you have a Ferrothorn weakness, Rayquaza swaps out its special move for Overheat/Flamethrower instead of running two of them. A case could be made for running both special moves if you're running Ray/Xern but you generally don't need to run both. Physically based LO Rayquaza typically does a better job due to OHKOing 0 HP Kyogre and being able to OHKO standard Xerneas after minor chip damage (or with no chip damage if you're an RNG god and get the ~20% rolls every time). Extreme Speed is also such a useful priority for being able to bypass redirection and offering reasonably strong chip damage in an emergency, and since Rayquaza hates Trick Room unless it's like up against +2 Xerneas or a Mega Salamence the utility that Extreme Speed offers is usually too good to pass up (we're talking about an ESpeed that's only ~5% weaker than the same ESpeed from Ekiller Arceus).
For the special slot I'm a bigger fan of Draco Meteor since it's arguably the best general use move there though if you really need the Ferrothorn answer, I'd drop Draco Meteor for Overheat/Flamethrower. Alternatively consider adding another Ferrothorn check like Talonflame, Infernape or some other Fire and/or Fighting type, but otherwise remember that Mega Rayquaza Dragon Ascent still does 75% minimum to common Ferrothorn (252 HP / 0 Def) because Mega Ray is stupid strong.
For the special slot I'm a bigger fan of Draco Meteor since it's arguably the best general use move there though if you really need the Ferrothorn answer, I'd drop Draco Meteor for Overheat/Flamethrower. Alternatively consider adding another Ferrothorn check like Talonflame, Infernape or some other Fire and/or Fighting type, but otherwise remember that Mega Rayquaza Dragon Ascent still does 75% minimum to common Ferrothorn (252 HP / 0 Def) because Mega Ray is stupid strong.