I can share a few thoughts of mine for a center channel. I dont feel it has to perfectly match your fronts as far a bass response, but it does need to get lower than most comercial center speakers do and I think the efficiency should match pretty close.

I made my own horn center speaker and its a noticable improvment over a yamaha and a jbl I had used. I wanted to keep mine small so i used 6" woofers, which dont give me quite enough efficiency so the horn is padded a lot. still the whole rig is much better than others i have had. I have an eye out for some higher efficency 6" or i may just switch to an 8".

the horn brings out a lot sounds for the center which is nice to have. Although i'm not a big fan of lpads. I have one on this speaker which lets me tune it a little for each movie. a mono rack EQ mounted into the back of the speaker would be the nuts, but thats another project! perhaps a bit overboard too :)

anyway, the closer you match the better. you should get down to 55-60hz if you can. if speaker size isnt an issue for you, then it wont be a problem, just build a 4 Pi into a TV stand and be done with it!

Bill