Will a clustered index improve performance of updates performed sequentially on that clustered index?
The IN like this is a set of discrete values. SQL Server won't bother working out it is a range or checking ascending values.

That is, x IN (1,2,4,5) is parsed out to x=1 OR x=2 OR x=4 OR x=5. OR is non-SARGable in most cases and result in scans rather than seeks. 

If you put the data into a temp table with an index and did x IN (SELECT foo FROM #bar) then it becomes a semi-join and will more likely use an index