Tested on Oracle 11g and as I expected, I could no reproduce the stalling behavior. An interesting tidbit is that Oracle generates a SQL statement like the following:

WHERE (ITEM_ID,PLANT_ID) IN ((?,?),(?,?),(?,?),...)

which the equivalent SQL Server's clause is:

WHERE ((ITEM_ID=? AND PLANT_ID=?) OR (ITEM_ID=? AND PLANT_ID=?) OR ... (ITEM_ID=? AND PLANT_ID=?))

I am curious if the following might be a more efficient SQL Server equivalent on 2008 and up:

SELECT *
   FROM EntityClass AS T
  WHERE EXISTS (
    SELECT *
       FROM (VALUES (?,?), (?,?), (?,?), (?,?)) AS V(ITEM_ID,PLANT_ID)
     WHERE T.ITEM_ID = V.ITEM_ID
           AND T.PLANT_ID = V.PLANT_ID
  )

This avoids the 'OR' operator and from what I have understood performs better but have yet to test it. I'm trying to see if there is any logical way to create such an EXISTS expression but so far I haven't found one yet.