Well, the more heat that can be dissipated from the internal hardware, the more you can push them to their limits. What I understand is that this BTX layout will allow better airflow than an ordinary ATX layout.
Also, it's not that the formfactor will have more processing ability, but more like that it has the potential to be able to push more out of it. Like you said, it's like moving furniture around, only so the stereo can pump out more effective sounds. ^_^