Scenario one. You have an 8" midbass driver in a box
crossed over from 70hz to 500hz.

Scenario two. You desire to replace that 8" inch driver with a
12" midbass driver but don't have the space on the front baffle of
the box because there is only 8" width available, so you compromise
a little bit and you recess the 12" midbass ~6" to 12" inside the box
and taper the wall so the energy squirts out of the 8" opening on
the front baffle.

Questions are;

How much senstivity will you lose, isn't this sorta like a reverse
horn effect sensitivity-wise?

How much more "punch" in the music do you gain, if any, vs. the 8"
driver?

You wouldn't really gain anything going from an 8" to a 12" midbass in this application. It won't necessarily sound any better, although the 12" would have more output capability because of the increased cone area and likely excursion as well. Still, a good 8" midbass, in the right box crossed at 70-80 Hz should be able to hit 105-110 db without overexerting, so it depends on your design goals...

Why do you want to switch? Maybe you could use two 8's?

Adam

Placing a board to cover the opening (at a 45 degree angle)
about 1/3 to 1/2 just seemed to suppress the SPL and alter the
freq slightly. Appears that I choked the bass output - haha
*BUT* it was still louder and more punchy than just having
the box sit there in open space.

Hornloading does rule, too bad I don't have the room to make
what I really want to make. There is no magic bullet.

For now, I will stick with my traditional design, perhaps someday
make a horn loaded system. hahaha

I do like the chest pounding midbass from concerts.