How many of yall would power up an A/A ranked legacy moveset if the IVs were under 90%

If it's your highest IV and already level 20 or above, do it.
Anyway, why do people consider IVs between 80 and 90 low? Even 75 is high for me, though I still aim for the highest. Finding high IV pokemon isn't a problem for me, as nests are everywhere here.

What are the exact IVs?

With zero IV, that is still a better attacker than most other pokemon with 100% IV.

I would. As long as I love using it and it's relevant, why not?

I have a 93% Jolteon with TS/T, and I was wanting a Jolteon with TS/TB. To make a story short, I caught two Eevee's out in the wild at 78 and 80 pct, and I evolved both into TS/TB. TB is so much better than T, that I have powered them up equal to the 93%. All now at level 37, I would use the 78 + 80 way before I would use the 93, simply because of the move set. Obviously, this happened weeks before the TM's came out. If your 84% has 15 attack, I'd really consider powering it up.

Don't waste on low iv ones!

In this case, IV's REALLY don't matter. An 84% with that legacy moveset is better than a 100% E/SoB IMO. Power that sucker up and reap the damage.

I would base the decision on how likely I am to get a better mon over a reasonable time in the future. I'm basically not seeing Omanyte/Omastar right now, so what I've got is probably best I'll get. Eggy is still coming up, so I'm not powering anything up unless it rocks the IVs. Well, that and stardust. Got lotsa dust? Power more up. No dust, be more selective.

Not sure what you want to accomplish with this move combo.
For attacking I prefer either Bullet Seed/Solar Beam or Confusion/Psychic. Especially with the buffed type effectiveness.
And for defense Confusion/Seed Bomb.

Even if P2P trading ever comes into the game sub 90% legacy would be virtually worthless.

TM that mutha uckah!

The reason it isn't worth powering a sub-90% active moveset is the prospect of getting a better IV Mon in the future. With legacy sets, you've got what you've got. If the only Eggy I had with confusion was 0%, you better believe that I would take him to L30.