Worth powering up/ maxing out?

Just transfer it and delete your account. Im baffled that you made it this far. Seriously read up, do research before asking a dumb question.

What the first responder said. Except exactly the opposite.

IF you have the Bagon candies, do it! If you DON'T have the Bagon candies, walk it. Don't use Rare Candies on it unless you have no realistic opportunity to catch a great Latios, or you don't have a Rayquaza.

That's the only thing potentially wrong with powering up a Salamence - using Rare Candies on Salamence when those could be used on Legendary Pokémon instead. Other than that? A 98% Salamence is AWESOME.

So the real question is:
Are there any relevant breakpoints that a level 40 Salamence can hit with 15a IV but not with 14a?

I haven't checked Salamence yet. You could look up how to check breakpoints and look at the future T5 dragon raids.

Mine is 15/13/15 and at 3312 til I hatch another Bagon... it maxes out over 3500, if it didn’t then I wouldn’t be as happy to max it (when I can find the Bagon candy).

Dragonite is always better and way easier to obtain...

I wouldn't. If I'm planning on a long-term investment, I want the Pokemon to have 15 attack. Even if it doesn't make a difference with breakpoints now, it might make a difference down the road.

If you don’t have enough Dragonites or a good Rayquaza, do it, dragons are the best counters against Lati twins and are very good generalist, if you have 3 or more maxed DNites don’t power up it