During porting, we are noticing that many a time, a subset of these things happen (a) implement similar-looking functionality in all subclasses (b) we sometimes forget to implement the feature in extension/src/* (c) parser test and parse.php both test slightly different versions of SiteConfig and neither of which test extension/src/* which is the production version.
Of all these, (c) is perhaps the most concerning. While unit tests and roundtrip tests help us with this testing of the production version, we should nevertheless investigate to what degree it is possible to push the abstraction and move more functionality out to the parent class by relying on the DataAccess class to capture the variability. This will require some mapping objects to hide the variations, but could overall improve the testability as well keep the versions more in sync.
I don't expect substantial changes, but more tweaks to the SiteConfig interface to add more base class methods that rely on additional DataAccess methods to hide the variations.
This is not something for now, but potentially a post-port consideration.