I would think its Horse Racing hands down. I had a friend DBA who worked at a company that worked with the breeding aspect of racehorses and their data...
"This comes after a history of patches that haven't installed correctly, patches to patch patches, and then patches to patch the patches that were released to patch patches."
If the queries you mention are run so often that this is an issue then why not keep the original identity key and add the new varchar() column and add...
How about going a roundabout way to do this? For example if its always a report for the current day, create a procedure that runs on the slow machine...