Hi all. I purchased a Retina iMac 5K and have basically tricked it out - 3TB Fusion, CPU upgrade, GPU upgrade. I have however ordered it with stock RAM with the intent of upgrading it using third party RAM. My question is this. Will buying a 16GB kit (2 x 8GB) and installing that alongside the existing 8GB (2 x 4GB sticks) be non-optimal? Is it better to have a machine with all the same sized sticks or does it not matter as long as you have matched 'pairs'? I may in time upgrade to 32GB, but at this stage I'd like to give 24GB a whirl and save myself some money after what is quite an expensive purchase. Insight appreciated!