What to expect from adding a brass phase plug?

[FE3T]

I have the following drivers
Vifa P21WO-39-08
Audax TW034XO

My wish are to build a pair of Sealed 2 way speakers with a volume of 1-1,25 cuft.

But, The Vifa`s dont seem to be offering the kind of frequenzy response needed too do this with a simple x over.

So i wonder, will removing the dustcap and adding a brass phaseplug clean up the 1khz-3khz area on and off axis without adding other difficult artifacts?

I have also seen that some manifacturers use Copper or Aluminium caps or rings in the polepiece area to reduce distortion (and maybe also induktion)

Would a brass phase plug also help with this?

Questions basically boils down to this.

can the results of such a modifikation even be predikted, or are measurements the only way to even get close to real world specs