Comparing the sha-1 is much faster than reading the ssd, so it's almost free.
Accidental sha-1 collision is probably not a problem, but in a few years [1] it will be possible to crate sha-1 collisions and use that as an attack. It looks difficult, but supposes that with the correct string an attacker can retrieve the cached information of another user, for example sha1("joedoe:creditcard")=sha1("atacker:hc!?!=u?ee&f%g#jo").
I don't know if they are using randomization, because the collision can be used (in a few years) as a DOS atack [2]
[1] http://www.schneier.com/blog/archives/2012/10/when_will_we_s...