Electrical power (current) demand of the PIAA horns is about the same as the stock wimpy horns (in prior years when there were two) and it doesn't appear they downsized anything in the signal (horn) circuit when Yamaha decided to save 50% on horns.
I had no reservations about installing a pair of PIAA Sports Horns on my 2014 without relays. They are nice and loud.
Yes, you could add a relay harness. And yes, the horns will likely be marginally louder if you do that due tio the heavier gauge wires and lower voltage drop on them. But, you will be adding an electro-mechanical device to your otherwise simple horn circuit (power to horn, horn switch grounds the other side of the circuit). That means that the day may come when you want the horns to work but they don't make a sound because of the added device.
YMMV