Does implicit casting negatively affect index usage?
Such conversion often will occur, and they can often impact cardinality estimation (and hence can affect plan quality). It will depend though on data types, existing indexes, cardinality etc. For example this query might not be able to seek in any case, as SQL Server might be using a scan regardless.

You should be able to determine if this is an issue in your case by inspecting the execution plan - do you get a scan on eventCount if you first cast eventFactor to a smallint? What if you use a local smallint variable that would result in the same number of rows that would match the filter? We can't tell from here...