Just to disturb the peace a bit, no "mastering plugin" can be as dependable as dedicated OUTBOARD mastering equipment. My biggest gripe about clients wanting their mixes as loud as the new My Chemical Romance CD is a result of them not knowing that they need a dedicated mastering house with outboard equipment that can squish more signal into the dreaded 0dBFS ceiling. In a software scenario, your computer can process data only within that range. In a hardware scenario, you can shoot the raw mixdown a few more dB's higher WITH LESS DISTORTION before sending the processed signal back to a recorder with good ADCs. Aural Exciters and Sonic Maximizers also give better perceived loudness to mastered mixes.
AFAIK, mastering engineers abroad use computers mainly for de-noising, editing, and de-amplifying loud transients in the mix ONLY. The multi-band compression, EQ, reverb, and hard limiting is done outboard before sending them to a recorder (can be your DAW with better converters).
Not all mixes can be mastered very loud, especially if the mix in itself benefits from a compressed sound, quite similar to older analog recordings. But if the mix is very dynamic in nature, (i.e. loud kick transients, separated instruments, more hi-fi sound) it is easier to louden that mix in the mastering stage.