I just had a glance over Hella horns spec and this is what I found:
Hella Strong Tone - x2 48w (This is the stock horn fitted from factory)
Hella Black Twin Tone Trumpet - x2 72w (This is same as black pearl trumpet)
Source:
PDF
So the current drawn by trumpet horns is 24w or 2 amps more then stock. If there is already a relay fitted from factory then we don't need to worry but I'm finding the proof. I have fitted Bosch Symphony Horns without relay in my car and have not faced issues past 18 months but I'm still bothered about relay.
However, adding relay isn't a easy task and not a good idea for warranty applicable cars.