IMHO I would suggest to have an external hard disk for the instruments data formatted as non journaled.You would
make things even faster replacing the optical drive with an hard disk dadicated to sample content.I have done that to my mac mini not for sample content but for recording purposes .For the os partition I would leave it journaled
Well, since journaling is a sort of file system log that keeps track of changes to the disk directory, if you are not copying, duplicating, creating or deleting files whether it is on or off becomes a moot point. So since you ask in the context of playback only, the advantages of journaling outweigh any possible speed hit, so it’s a definite “yes” to leaving it on.
After reading articles at apple.com and other sites, (which is probably what you read too… there is not that much out there)