John is right because the tweeter has wide horizontal dispersion but relatively narrow vertical dispersion. I was thinking of mounting it vertically but I should have said so because not everyone is aware of how these things work.

As for how much you'd lose, I can't say because I haven't tried but I will over the next month or so since M5HPs are coming my way. smile My vote would be on not losing anything in your use case because you won't be pumping 400W into it.