what mid can you think of that's at even close to 100db effecient that WILL play down to 60hz and up to 1.2k. Pro audio drivers are iffy, even in an AP. By the time you ask a driver to play that wide range you've comprimised power handling. Regardless, loads of power is only going to do so much. You may end up needing to put your mibass out of phase to get the lower midrange to image as high as those horn (rainbow effect is a *****), in which case your midbass goes bye bye. By the time you pad those horns down to get them to the level of the midrange, you've almost defeated the point of the extra effeciency.
I'm not saying 2-way horn setup's can't work. They've been known to work well in the past, classic JBL drivers are legendary for their 2 way horn ability, but impossihble to find! I'm just saying if your going to the extra effort to use horns in the first place, might as well really let them loose, the other drivers are ALWAYS the weak link in a horn install. By the time you get them setup right you have a hyper effecient, well imaging driver with amazing tonal characteristics, hard to match with a regular driver. Especially when your asking 1 driver to do it. If you don't have room, money, or willingness to try it, cool, different strokes for different folks, it was just a suggestion.
Anyway, I'll thow a couple drivers out
Dyn 170's
IDQ 8's
CX62's)
Dayton RS 7's
Morel WR-8's
B&C NDL51