Efficiency at any Depth of Aggregation
Aggregation has one interesting aspect when aggregates are used on more than one level of an object implementation. Imagine that the TextImage object in the previous example is itself an aggregate object that uses other inner objects. In such a case TextImage will be passing some controlling unknown to those other inner objects. If TextImage is not being aggregated by anyone else, then the controlling unknown is its own; otherwise it passes the pUnkOuter from IClassFactory::CreateInstance on down the line, and any other inner objects that are aggregates themselves do the same.
The net result is that any object in an aggregation, no matter how deeply it is buried in the overall structure, will almost always delegate directly to the controlling unknown if its interface is exposed from that final outer object. Therefore performance and efficiency of multiple levels of aggregation is not an issue. At worst each delegation is a single extra function call.