I am just about to buy a 13" rMBP (I think). I need 512GB of SSD space, so I am looking at the top-tier model with either 8GB or 16GB ram. Does the decision have any performance implications with regard to external monitors? If I hook up a 4K external display to the machine, will the 16GB model handle the load better? I am asking because there is no video RAM in the model, so whatever amount of memory is required to power the external display will be deducted from the available RAM. Many thanks.