Evolve larvitar

Can you do tyranitar raids? Every 15 atk tyranitar from a raid will be better and you will save 87 candies. By evolving you only save 40k stardust.

Low def isn't that important. Does it make a difference? It does. But is that difference significant? Not really.

The def is low enough that it can't reach the 9 damage breakpoint for Confusion against Mewtwo even if maxed out but that is probably never going to make a difference in whether you win a Mewtwo raid or not because there'll be plenty of people who show up to Mewtwo raids.

Difference in Shadow Ball damage taken between it and a perfect one is only like 3-4 points of damage. Focus Blast doesn't matter because it'll one shot it with 15 def anyway.

I think the biggest reason not to evolve it is what gluglumaster said, it'll cost you more Larvitar candies to evolve this one than to power up a level 20 Tyranitar from a raid up to this Larvitar's current level, 26, and if it's not going to be better than a Tyranitar from a raid, what's the point in using ~90 extra candies?

That assumes your options are not limited and that you can actually do Tyranitar raids though. If evolving Larvitars are your only option, then I would say to evolve it, compared to an egg hatch it's not going to have a significant difference in perfomance but you'll save yourself about 30 candies in power ups, and it's unlikely that you'll find another wild Larvitar that is has both a high level as well as good IVs.

I also do that to save stardust - find high CP mons with high IV attack & Stamina stats (ignoring a low or non-existent defense stat) and evolve them as fighters.
They do really well in battle, and save me a lot of stardust. And if I'm not going to use them as defenders, why not?

Just go ahead. I evolved my first two Tyranitars from high level Larvitar and Pupitar respectively. Both are 87 % att 11, usually considered not worthy of spending dust and candy after raids came along. They have been magnificent for my team and recently I managed to get a third in one of the few raids I’ve participated in. 82% att 15, took it to level 30 straight away and now I can solo Gengars. If you want perfection, don’t do it, but you will not have as much fun on the way to the perfect team.