As you said

"... The only danger with this solution, even though the chances are very slim, is that the different original values might produce the same hash and result will be wrong .."

Then, Why NOT just use ?

 CHECKSUM_AGG(BINARY_CHECKSUM(*))

That, is going to give you a real performance boost

ex:

SELECT

(SELECT   CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM processed_customer)

 -

(SELECT   CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM customer ) /* 0 means OK*/

Just my $0.02