It really depends on your performance characteristics as you mentioned, you can certainly make a database fast as hell but most of the time I would recommend SQL for when you need ACID characteristics(Atomicity, Consistency, Isolation, Durability), not performance per-se because while SQL can be insanely fast, if you do not understand the abstraction it can get slow real fast.
If you find yourself storing large tables of information that is vaguely relational and you value the consistency of your data over raw performance I would generally recommend SQL engines.
I guess i'm more envisioning something like a SQL container. So if you have multiple instances of a pImpl class you would keep the actual state-variables/implementation in a DB. When you want to say do some complicated look up (ex: which objects fit some criteria) you would express that operation in terms of SQL queries
If you find yourself storing large tables of information that is vaguely relational and you value the consistency of your data over raw performance I would generally recommend SQL engines.