Dynamic Headroom vs Resistance

I'm in a discussion on another board and a member has asked if he adds 2 more speakers onto his amp (cutting the ohm load in half) will he get a louder, more dynamic system with better headroom?

I was under the impression that
the fact that your powering 2 extra speakers, and drawing more juice outta your amp means you will reach clipping faster as your amp runs outta juice, so you will lose your headroom because your amp will be pushed harder.

to which he responded:
There's more power available. Why would the limit be reached faster?
Half the load means double the power. Wouldn't that increase headroom?
You'd be using the same power as before to reach the same volume level. Only now you're using a smaller percentage of the available power.

Now figure in the extra speakers. The amp needs to deliver even less power than before to reach the same volume because there is twice the cone area. The percentage of power is smaller still.

According to Stereophile, "Dynamic Headroom," is the ratio between the maximum power the amplifier can put out for a burst just 20ms long, and the continuous power. An amplifier that can deliver 200W for 20ms but 50W continuously will have a dynamic headroom of 6dB.

What am I missing here...
is he correct that you'll get more headroom?
but this has no impact on transiet response correct? as thats a seperate function?
sorry if this is confusing...
Geoff