Denon53, you might want to start a thread about phase plugs. What I can suggest is:
1) Seas likes them because it reduces air pressure build up behind the driver so it moves more freely. May not be important if the x-over point is under 1,500 Hz.
2) I think the voice coil is NOT attached to the driver cone so it is lighter.
3) The phase plug is designed to reflect sound of the inner diameter of the cone so you get more even dispersion as the frequencies increase. Normally dispersion is less and becomes more directional as the frequency increases.
I have directly compared Seas Excel W12CY003 which has a phase plug vs. Seas Prestige CA12RCY (solid cone) and even though the
measurements Seas Excel W15LY-001 (Nextel) vs. Seas CA15RLY like harmonic distortion appear worse with the phase plug driver, the Excel sounds noticeably better. It is more dynamic, more detailed, and more alive sounding. This has happened generally across the board with commercial speakers like Revel, Focal, B&W etc. But since the Sierra is over twice the price, maybe it is better in spite of the driver's inherent shortcoming, IMHO.
Again, dome midrange are a whole other league and worth asking about if at least for information's sake.