[[ The M500 is a good choice if performance isn't your top concern. ]]
I'm currently using a 240gb m500, mounted in a plugable.com "lay-flat" USB3/SATA dock, as my "external booter" (late-2012 i7 Mini).
It's giving me read speeds of 276mbps and write speeds of 432mbps.
Maybe not the ultimate, but not bad at all.
I'm assuming you got those speeds round the wrong way..