My zapdos

You get your breakpoint against which defender? You realize that there is not just one single breakpoint, correct?

I don't understand why you are asking if it is "bad" to power this one up. It's your pokemon to do what you want with. Some people are adamant that they will not power up anything under 91% or whatever, which is silly, because depending on the IV distribution, certain 89% mons are better than 93% versions of the same species. Others will power up a 15/10/10 (A/D/S) mon, but not a 14/14/14 one. Other people will tell you that no legendary bird is worth powering up. Fine, they can do what they want.

Personally, I'm going to power up my best example of each bird after they are (temporarily, I assume) no longer available. Right now, my best Zapdos (out of 3) is 80%, with a 15 ATT IV and the best moveset. If that ends up being my best one, I'm fine with powering it up. The difference in both CP and practical usability between that and a 91% one will be negligible (like <1.5%), and since they can't be put in gyms, the CP difference is not meaningful.

Bottom line, do what you want!

I would power up the one with high IV in the stat that can compensate for its base stat. So as for Zapdos, i would much prefer to power up the one with high defense and stamina IV.
Let's do some calculation(comparing to 15 Attack IV):
14IV attack would only decrease by 0.3745%;
13IV attack would only decrease by 0.752%;
12IV by 1.13%;