The difference in a CD or an MP3 after it comes through a comercial PA is so minute that I would never loose sleep over it. If a midi file is recorded at a quality level and converted to MP3 and played back on good gear ... the result will be VERY, very good. Will it be the same? maybe..... maybe not.
Some of my backing trax that I made for "production" numbers include backing vocals and sound so much fuller and better than any of my midi files. It is a case of the sum of the parts is greater than the whole.....or some analogy like that.

I have been using MP3 and WMA files from my laptop for years now ... and given the choice of lugging a case of CD's around or just hitting the "enter" key ....hmmmmm, lemme think about THAT one for a sec. Nope ... in my situation ... the MP3's provide a viable alternative to having a rack of gear and a roomfull of singers.
Some of my really high bitrate MPs' sound everybit as good as CD's through a PA ..... remember, it's not a perfect, studio setting on the job. The difference is SOOOOO soooooo slight, and almost nonexistant. If you hear a big difference, than you should RE-RECORD your MP3's cuz mine sound awesome.
If the initial recording is good ... end result will be good too.