@Steele:

Maxing affection doubles the bonus received, which is why even if your 2* is unevolved, her stats can compare to 5*s of slightly higher level. At 40-50% affection vs. Max affection, the bonus your 2* is receiving from that is essentially 3.5-4x what the 5*s receive (affection bonus is linear, so 50% is twice the bonus of 25%, so 100% would be twice 50% except the doubling makes it 4x, slightly less due to higher rarities getting bigger base bonuses.

As Maotd mentioned, if your 2* is evolved, that will result in a substantial power advantage over a similar level, but unevolved 5* (since it's essentially 40-50 extra levels). Another thing to keep in mind.

In the end, if you're a power gamer/optimal setup type guy, you're going to ditch 2/3*s and eventually 4*s for the 5/6*s due to superior passives in addition to stats. Roentgen went over the advantages 5* event girls have over 2/3/4*s and even have some ways to balance their statistical/passive inferiority to their 5* gacha counterparts. The reason you'll want full 5/6*s is Passive ability synergy. This is ultimately the deciding factor of who makes the team and who doesn't (unless you whale hard enough to get lots of 6*s who auto-kick out the 5*s and below) because synergy can drastically boost a team's effectiveness more than simple leveling (and of course you'll eventually hit the level cap on your girls).