Hey David. Seems that neither of us consider a horizontal trumpet as a first choice. Of the many organs I've heard, either in person or recording, I've heard many of these which I've cringed or laughed at, and have thought to be inappropriate in their settings (esp. in acoustically dead spaces). One example I never cared for are the Boisseau Chamades at Notre Dame de Paris. At times, they're like nails on the chalkboard.
Of the few chamades I do like, my favorite by a long shot is the State Trumpet at St. John the Divine, NY. Another is the one at St. Thomas' Episcopal, Fifth Av., also in NY.
What makes the Horizontal Trumpet seem louder than a Trumpet isn't necessarily so much in the decibel level, but the direction in which the sound is directed. It's also the fact that there is a larger number of harmonics heard from a trumpet pipe pointed towards the listener than one that is pointed at the ceiling. Both of these pipes could be identical. The same make, same pitch, same wind pressure, same voicing....
Even a moderately loud trumpet mounted horizontally can have an impact with the additional harmonics heard as a result of this positioning. Randy Gilberti, former curator of the organs at St. John the Divine, told me one day that the word chamade is French for startle.
Imo, caution should be used in including a chamade in a design.
Tom