In which cases are sha1 used now? If not common, maybe it could be partitioned vertically, also making easier to change the hashing method in the future? Not sure about this, just a suggestion as soon as I saw the sha1 still there.
Topic on User talk:Brooke Vibber/Compacting the revision table round 2
AFAIK sha-1 is unused and was only added because it seemed like a good thing to have that we might use for something in the future. One use case that came up is revert detection, but I think we might be better off with an explicit field for that, which I'll start a separate topic for.
I use it for comparing the current Wikipedia database with old dumps to find missing edits. See my user subpage on the topic. This work would not have been possible without *some* sort of hash field ... the actual hashing method used would be immaterial, AFAIK.