I understand that you have too much new information the last days but please UPDATE THE Gym Attackers Tier List
Answers
Why do you even need a tier list for attackers? I rarely power up attackers, as you can choose the matchup, you don't need them to be as high level as the defenders. The other thing is... for attacking, type matchup is much more important, so the main reason to choose your attacking team should be the type matchup (except against snorlax and lapras I guess), for that, you are better off using the highest DPS per type page.
I generally max out my attackers (or at least to lvl 30) so battles are quicker; because they get beefier, too, they take less damage which = less healing = less resources used = overall higher efficiency. Since you don't gain more xp or take off more prestige from handicapping yourself, best way to go is to use your strongest.