An IV question - how important are A & D compared to H

Honestly for DPS, attack is all that matters.

Some people argue that 15 A/H and 0 D is optimal because you'll take more damage and thereby gain more energy faster and get your charge moves faster - however except in very rare circumstances where you exactly finish off a charge move before dying, this really isn't the case as the extra defense IVs making you survive longer will net more total DPS from charge move reliability and less time fainting out.

HP and Def ultimately don't matter much if it doesn't affect a bulk point for you (where you can survive one more charge move of your enemy - although to me a better bulk point would more be where you can survive long enough to do one more charge move yourself...). However I wouldn't bother powering a Flareon - it's already outclassed by Moltres / Entei / BB Charizard, and will be outclassed by more in gen 4.

If you don’t have any Moltres or Entei to power up then I’d say yeah. I don’t think that Charizard really outclasses him in DPS and he works pretty well. But again, that’s only if you really need a fire type. I mean he is outclassed and will be in the future but does that mean you should wait potentially 2-6 months for that to happen? No, you can simply power him up and he will do well for what you need imo.

Flareon has a stamina stat of 130. A 100% Flareon would have 11% HP more than yours. At level 30, yours will have 95 HP and a 100% would have 106 HP. I guess it could mean one or two more quick moves sustained from a raid boss? You can try to fight with it to estimate the loss you suffer at each quick move.

While I think about it: if you are in lack of Registeel counters you could power up your Flareon, but otherwise I would advise to wait until Eevee community day. What will happen with moves is still not clear and you might get a better Flareon anyway.