Are Legendaries with 89 iv and 15 attack worth to invest in ?

Yeah man, functionally a few ivs will not make much of a difference. Being able to dodge efficiently with those ivs will perform better than having 100% (?) ivs while dodging less efficiently (in my opinion).

For me, i prefer to have mons that have 91%+ ivs (OCD hahah) and at least one stat that is 15 ivs. I have a 89% articuno and a 91% 13/14/14 Lugia and since neither of them meet MY criteria I will refrain from powering them up. Both however will perform just as well as their counterparts wth a couple more ivs. And if Dragonite is released as a raid boss before articuno is released back into circulation, I may just power up the one I have

A friend caught my first Lugia yesterday with 89%ivs and 15 attack (15A/13D/12S). I will TM and power it up if i dont catch a better one (its Es/HP and i want Es/FS).

Wait until event is over and you can decide. If there are no leg birds at all and you want a strong one, you will have to power up the best one you have got.

If they will still occur from time to time, you could still hope for a better one.

Don't power up articuno
Only power up Moltres if you're looking for a better Flareon
Only power up Zapdos if you're looking for a better Jolteon

Always power up Mewto

They are legendaries and IV's are horrendously overrated.I will power up my best of each legendary to L30 even if it has mediocre IV's (my best Articuno out of 5 is a 82%).

If it's moltres, you still have a chance to catch a better one, but if you don't, it's still a fine choice, however if it's Lugia or Articuno, they wouldn't be worth powering up even if 91%+ IV's.