How much power is to much power?

It depends.

On room size, how loud you listen. That's about it.

Realize that power alone is not always the answer. The amp has to be able to deliver power smoothly into the speakers variable impedance at different frequencies. Amps that double in power from 8 to 4 to 2 ohms do this best. Some speakers are more difficult to drive due to complex impedance loads at various frequencies. Others are less demaning.

My understanding is Df matters but there is a wide tolerance in values that will work in any particular application.

You have to listen and judge. At least with dynamic drivers, if the bass sounds mushy or soft, a higher damping factor may help but the actual value does not matter that much.

"When is an amp overkill for a speaker?"

I guess the simple answer is: when it overdrives or fries the speakers.

Practically, however, in most cases fried speakers are a result of not enough power resulting in clipping. And, if the speaker is being over driven at a volume you like, then get another pair of speakers.