I'm looking to get a 15" retina. I want 16gb of RAM (currently have 20gb in my iMac and use 12-13gb constantly in normal use, often more) but I tried the 8gb in store and it was ripping fast despite the base amount of RAM. A refurb 8gb RAM 256gb drive is about $1700, while a new 16gb with 256gb drive is about $2100 (education discount). Is the $400 worth it for the extra RAM? Thanks!