I‘ve got my retina MacBook Pro (2.6 GHz, 16 GB RAM, 512 GB SSD) and a bus powered Thunderbolt Elgato 240 GB SSD today. The retina display is incredible. You‘ll never want to go back.
The new retina-optimized FCPX plays a 4-minute 4K ProRes 4444 FCPX project in 50% (2K) fluent from the external TB SSD and there is still enough space for menus/timeline etc. Everything looks much better than on my late 2009 27“ iMac.
FCPX export from 4K ProRes 4444 to 4K ProRes 222 LT takes 7:55 on the MacBook and 12:26 on the iMac (16 GB, 2.66 GHz i5).
Copying the 10.5 GB video file from the external Elgato SSD to internal Macbook SSD takes 0:42 (250 MB/s).
Btw, OCZ has announced a much faster (500 MB/s+) external bus powered SSD (Lightfoot) that should arrive in a few months.