Yes to everything. IIRC, Mac OS is a derivative of FreeBSD.
http://unix.stackexchange.com/questi...os-x-come-from
http://en.wikipedia.org/wiki/OS_X
Chances are good that it's the HT, as you surmised. The AMD wouldn't have that, so it wouldn't have to share cycles with other tasks.
Actually, I'll also throw out there that the App simply may have optimization issues for the Mac as well. May be it wasn't compiled or ported well. Just like the Linux variant of NFS used to outperform the Windows version significantly, that kind of variation is not unheard of in cross-platform environs.