In my simulations of a 20"x10" horn(vs 24x12 for the Pi horn), all the horns in the 180-200cm2 throat range looked good. With a spec'd Sd of 220, I imagine a throatless 8PE21 horn would be almost as good. One question of mine has been how to create a throatless midrange horn, since 220cm2 is quite a bit smaller than the spec'd 8" diameter. Therefore, you'd still need to create a 'throat' of sort to cover the driver's surround.