I upgraded to OS X 10.10 Yosemite about a month after it was released, my usual waiting period. Before the upgrade I maxed out my mid-2011 27” iMac to 16 GB and have not had any memory issues so far. LR works just fine; I don’t use iPhoto. My images are stored in a WD RAID. I intend to upgrade to El Capitan in due course. Oliver