OK, I know this is already QCed, but 252 Def / 252 Spe is an absolutely ridiculous thing to have in AC. Let's look at the reasons you've provided for using it.
Jirachi: firstly, no worthwhile team is going to have Jirachi as the only possible way you can set up a NP Thundurus. So that means it's reasonable to assume that you can just NP on something else, which doesn't require 252 Def EVs to do so against. In which case, by the time you actually see the Jirachi, you can OHKO it. Plus, you get the added benefit of having a Thundurus that's actually useful if it doesn't get to set up.
Ferrothorn: it cant Thunder Wave you. Gyro Ball does laughable damage as long as you aren't using Substitute against it. You can OHKO it with LO / 2HKO it after a boost otherwise. Literally all it can do vs you is Leech Seed, and do you really care about getting seeded with a NP Thundurus? Once you NP, one of two things are going to happen: you're going to outspeed and kill everything, in which case Leech Seed doesn't get a chance to do anything, or you're going to get outsped and forced out / revenge killed, in which case it doesn't matter either way, and you should have just killed the Ferrothorn in the first place.
Really, wanting to mention a Thundurus-T without any SpA investment is just silly.