Very surprised someone is booting routinely from external drive. If I had old Macbook Pro with HD, I would would replace it with internal SSD. For a rMBP 2012, Transcend now offers replacement SSD for SATA. And we can get fast 1TB PCIe SSDs in 2013 rMBPs.
My use case is likely unusual. I have the 840 partitioned to be bootable in an emergency. The large partition is for Time Machine backups of the internal 768GB SSD when I am on the road. I don't care about trim. When I get home from a 2-3 week trip and move the photos to my permanent library drives, I can reformat the Samsung without any problem. So for me, trim has no serious advantage and an external power supply is a big negative.