So there are a number of huge architectural disasters which come from business logic in the DBMS. So I would avoid this like the plague. Been down that road a few times and while in the short term you gain huge performance you lose all long term maintainability and reuse. Bleh!

But, this is not to say that you cannot take advantage of the DBMS for optimizations of simpler CRUD operations especially in situations of Bulk updates involving related objects. That is a good thing but for the type of problem you are facing I think you are choosing the wrong tool.

Data Synchronization can be handled in number of ways. I would not recommend using the API to do very large bulk operations record by record. We have used Talend and other open source toold to do synchronization on large data sets distributed across numerous heterogeneous databases. This type of issue is not relevant to CiviCRM, though. So it should not be a consideration in the architecture.