Should SQL code be considered “code” or “metadata”?
Code.
Should stored procedures be used only for performance optimization or they are a legitimate abstraction of the database structure?
Stored procedures allow for reuse, including inside of other stored procedures. This means that you can make one trip to the database & have it execute supporting instructions - the least amount of traffic is ideal. ORM or sproc, the time on the wire going to the db & back is something you can't recoup.
ORM doesn't lend itself to optimization because of its abstraction. IME, ORM also means a lack of referencial integrity - make a database difficult to report from. What was saved in complexity, has now increased to be able to get the data out in a workable fashion.
Is performance a key factor the decision? What about vendor lock-in?
No, simplicity is. Vendor lockin happens with the database as well - SQL is relatively standardized, but there are still vendor specific ways of doing things.
