I haven't posted in ou in months so please don't kill me if this is a bad nom even though I think it's reasonable.
Thundurus-T C -> C+:
First off, while the opportunity cost might be pretty big, but unlike mega Latios, Thundy-T is actually a really solid pokemon. I think that we all know what this guy does best but if you do not know, Thundy-t is a really good double dancer aka it can annihilate nearly every team archetype at will (can also run only agility or nasty plot but I always found double dance to be the best thing and nasty plot seems pretty outclassed by regular thundy). Another flaw that is bound to be brought up, is that Thundurus-t is really pressed for moveslots, and I completely agree with this. Unfortunately is probably going to expect HP Ice which is usually the best move, but it can run HP flying, Focus Blast, or Grass Knot, it all depends on what you want to beat. The final thought I can think of is that Thundurus is pretty easily countered, and I will concede that this is pretty true (although pairing it with Talonflame can remedy this), however, you can beat a lot of mons with the coverage if you're bluffing hp ice. Remember I am only nomming it for C+ but think of some of the other C+ mons; Camerupt-mega, I would say that Thundurus-t is at least on par with it in terms of viability, Staraptor is pretty meh now because birdspam isn't
as viable but still is a valuable member on a birdspam team, and Medicham-mega comes with an incredibly high opportunity cost, much higher than Thundurus' because it is a luxury mega although it is still pretty solid.
Now that most of the cons when using Thundy-t have been stated, let's go over some of the pros when using him. As previously said, Thundurus-t can decimate any playstyle at will (assuming that you are setting up on something that will probably switch out or just hit Thundurus with a weakish move), aka you can click Nasty Plot, Agility, or both. Another great trait that thundy has, is not being
completely crewed over by priority because it resists Bullet Punch and Mach Punch, unlike many sweepers in the ou metagame. To go along with this, you can't use thundy-i as a safety net vs this sweeper, yet again, unlike most sweepers in the ou metagame. While on this topic I'd like to point out how great Volt Absorb is, albeit situational, but it allows Thundurus-t so set up on pokemon like Magnezone, Magneton, and Zapdos. Its speed tier of 101, while not anything to brag about, is a great benchmark, outspeeding all max speed base 100s as opposed to risking a speed tie with them. Also I have less lines in pros because they are a little bit easier to explain.
This is my example of a replay utilizing thundurus-t:
Even though it wasn't against the best players, it's still something.
TL;DR: Thundy-t is a really solid mon, while having a lot of cons, has a good amount of pros such as access to boosting moves, resisting/being neutral to all priority along with being immune to Thundy-i's Thunder wave that can bone so many sweepers.
Please be constructive if you are against this, along with not being too aggressive about it in your post.