You will find that continuity amoung the four speakers is not necessary. The surrounds are only for ambiance and effects.
Matching them all close in efficiency would be good.

Building (4) 2pi's to use as surrounds and a pair or Rear centers would be too cool for school. (4) 1pi speakers would be great.

You need to build yourself a very pristine center speaker. I would bet 50% of the movie comes from the center. you want your dialogue and main scene effects coming from the center to be crystal clear and dynamic.

Next your Mains are going to unleash the brunt of the score and your 4 pi's are perfect for this. It doesnt get much better.

I have a pair of 2pi speakers and I use them for my surrounds now.
They are Perfect. just the right size, just the right SPL, just the dynamics. The 2 pi's replaced my JBL 4655 speakers for surrounds.

I have the room, so I use 4pi speakers for the from mains. Since I have the room for the 4pi's, I wouldnt switch them out for a quad of 2 pi's

Bill