2011 iMac user here. I am moving to a MBP for the sake of portability. I do heavy video editing and sometimes audio production. My current iMac has 3.4 GHz i7, 20GB RAM, and AMD Radeon HD 6970M 1024 MB. It has no SSD. This iMac is simply not powerful enough for my video editing needs and I am guessing the internal HDD is the bottleneck. I am looking at purchasing the 15-inch Retina MBP. An extra $300 for 256GB more on the SSD is a lot of money for me. If I bought a 256GB Thunderbolt SSD and connected it to this laptop, would it be about as fast as if the drive was internal? Particularly for video editing. Thanks in advance!