Actually, that is what we did as well since we had to have a version that was backwards compatable to SQL 2000 but that we certified to run on SQL 2005. So that is not uncommon. Since it had to be backwards compatable we could not use any SQL 2005 features and it had to run compatibility mode 80.

Generally we find that performance over all is better on SQL 2005 but we do find several queries that run considerably slower on SQL 2005. We have opened up incidents with Microsoft on these and they admit there are some issues with complex queries involving outer joins. We have had to add some Force Order query hints to resolve them until we can rewrite the queries.