The speaker and headphone outputs are just different connectors coming from the same amplification circuit.
Yes, you get 60 watts for speakers quoted at 8 ohms impedance for speakers (if you connected 8 ohm headphones, you'd get 60 watts there, too).
Divide that by four, for 32 ohm headphones, and that gets you to your rated 15 W headphone output.
Other than a little bit of extra cabling to connect the sockets, and the sockets themselves, the output from the amplifier is the same for both the speakers and there is no additional circuitry involved. All you'd be doing is removing the speaker connections - and since that wouldn't get you anything sonically, just not using them gets you to the same result.