If MP3s are encoded with the right codec, and at the right bitrate, the sound difference between CD and MP3 is negligible. The high-frequency cutoff is a side effect of poor digital mastering, though that's arguable. Most CDs nowadays are digitally mastered louder than they used to be, and there is a smaller difference between loud and quiet, and frequencies above a certain point are cut off.
The CD will sound exactly the same as the leaked rip, unless it was an unmastered or pre-master or something. The conversion to MP3 won't cause a big enough difference to be noticed, unless the person who did the rip is inexperienced. I ripped all of my Melvins CDs to my harddrive, and they sound the exact same when played through the same sound system, using the same computer software.