The 12" woofer could play up that high to reach the open baffle mids but not with the same speed and accuracy. It would compromise the lower vocal region a little. It would cause more of a phase shift in the lower vocal region and split the 200Hz region into dissimilar drivers. I wouldn't.
As designed works well.
How about a deal on a fully finished and completed pair of OB-7 pluses built for use at the RMAF a few years ago?

Finish is oriental wood, oil rubbed. They are wired with Vampire Wire continuos caste Copper, Sonicaps, foil inductors, and even a custom bass management system on the four outer woofers.
$2,500 plus shipping and they are yours.