Type conversion in expression try_cast may affect cardinality estimate in query plan choice
Basically, this could mean that the type conversion cannot effectively use an index. Cardinality is a important property of the indexes that you might use.  For performance, the ideal is to have the data types match, instead of trying to convert them on the fly. 

You will find Thomas LaRock's post on this useful: http://thomaslarock.com/2012/08/why-datatypes-matter-3-ways-they-can-hurt-performance/