Well, the A chips have never been used in real world situations where the CPU was stressed for an extended period, like what you would have if you're rendering a 30 minute HD or 4K video.
It's possible the A13 (or whatever it would be in a desktop) would be cooler than an intel chip, but since they've only been used for dinky apps in phones, and none of those apps give it any real prolonged stress test, we have no idea what their actual thermal performance would be in a full fledged computer environment.