I believe Final Fantim has resampled the OST, not just converted MIDIs to mp3.
But don't let that stop you; the more versions of the OST we have, the better. It means more choice for people who, for whatever reason, don't care for someone's interpretation of the songs. Just to check: you're making a proper, orchestral style remix, right? Imagining how it would sound if Uematsu had had the Vienna Philharmonic to work with? You're not going to make a load of techno/metal remixes, are you?