I'm looking to get the 27inch iMac but don't really want to get a standard HDD. The fusion drive looks great but its an extra £200. I can't really find any infomation on this but is it possible to get an external SSD enclosure for the iMac and gain the full benifit from an SSD? I've heard USB3 is a bottleneck and thunderbolt is stupidly expensive. Is this true? .Thanks!