Externally-attached GPUs have been a topic for many years now. Numerous companies have tried, including AMD and Lucid, but no solution has ever been a widely known and available product. Even as interfaces increase in bandwidth and compatibility with internal buses, it has never been something that a laptop salesperson could suggest to users who want to dock into a high-performance station at home. At best, we are seeing it in weird "coin mining" racks to hang way more GPUs above a system than could physically mount on the motherboard.
Apparently that has not stopped the DIY community, according to chatter on Tech Inferno forums. While the above video does not really show the monitor, MacBook Pro, and GPU enclosure at the same time, let alone all wired together and on, it seems reasonable enough. The video claims to give the MacBook Pro (running Windows 8.1) access to a GeForce GTX 780 Ti with fairly high performance, despite the reduced bandwidth. Quite cool.