In a CASE Statement, which is faster: "WHEN [field] in (0)" or "WHEN [field] = 0"?
The following construct:

c IN (x, y, z)


Gets extrapolated to:

c = x OR c = y OR c = z


It will logically follow that c IN (x) extrapolates to c = x. Therefore the first comment is absolutely right: you will not see a difference in performance except at the very edge case (where the size of the query text is actually impacted sufficiently by the choice), but even there it is hard to fathom the performance difference being worth mentioning.