Most of the ~fullrange driver or tweeter WGs I've built have been this way since I didn't want any real throat pressure. BTW, a horn's or WG's throat (St) is the small end regardless of how big it is in relation to the driver's effective piston area (Sd). It just means that the compression ratio (CR) is < 1.0 when Sd is < St, so if a low CR works in your app, then it will work fine on a midbass too.

GM