this will get the whole world on my back but....
In my opinion, Phase plugs are a gimmick. just one more way to sell a speaker.
And, it makes the speaker less loud because it loses piston area and it means you have an open voice coil gap which could get dirt/dust/gravel in it and sustain coil damage.
The premise is that the plug breaks up the sound coming from each side of the cone.. the problem with that is the fact that the entire speaker acts as one piston.
so, my belief is that a phase plug does not do a damn thing.
The only way to know for sure is to buy a speaker with a phase plug and the exact same speaker with a dust cap instead and listen to them.