>I've discovered numerous SPs, in the system I inherited, that rely on implicit conversion of parameters.
>
>I've read that it is a best practice to explicitly convert these parameters, but I would like to know if there is a performance gain using explicit conversion.
>
>Thanks.

I really didn't test it, but for me if SQL Server should do some implicit conversion that will slow the query (with 0.000000(0)1 millisecond, just because that conversion is done once when the query is compiled.
BUT to relay on implicit conversion is not good for me. What if you pass illegal parameter?
Check this:

DECLARE @test varchar(20)
SET @Test = '0'

SELECT 1 + @Test

SET @Test = '0.0' 
SELECT 1 + @Test --BOOOOOOM

Against Stupidity the Gods themselves Contend in Vain - Johann Christoph Friedrich von Schiller
The only thing normal about database guys is their tables.