I run Logic 10.1 on Mavericks. I used Logic 9 for several years and i'm very happy with X. There was a small learning curve in the transition from 9 to 10 but that of course is per usual. It's my opinion you should definitely upgrade to X. However, I am using a 2013 21.5 iMac that is maxed out to 16 GB of RAM, 1TB fusion drive, and the fastest i7 processor you could get at that time. I write moderately sized musical projects of a wide variety and use a lot of 3rd party plug-ins. I rarely have to freeze tracks unless I use a lot of audio. I do not plan on upgrading to Yosemite any time soon, maybe for the life of this iMac. Hope this answers a few of your questions.