Well, the chip does get hot, hence the need for a double sided PCB with the top layer serving as ground plane and heatsink. The amount of copper used is sufficient to keep the chip within the operating paramaters, so adding another heatsink will add little value.
I'm guessing those amps are running in Class A? Headphones need so very little power to drive that the amp can remain in Class A for a lot longer than it would with a speaker. So biasing it up for full Class A operation seems a little silly. But then, using Dale instrumentation resistors may also seem silly to some, hehe!