Are there any performance benefits to using a hash table with no clustered index?
I would put a clustered index (not a primary key) on the sid column.  If you do lookups against the mid column sometimes as well then add an index to the mid column with the sid column as an included index.

If you don't want to create a clustered index, then two non-clustered indexes one on each column with the other column as an included column should give you the same basic read performance.