There will be some overhead, but it might be a wash considering calling over a FFI often involves similar overhead to marshall / unmarshall objects. And the simplicity gains would be massive.
In-process it's just function calls. Cross-process COM has automatic marshalling for standard types ("automation types") or you can define custom marshalling that does whatever you want.
WinRT [2] is a more modern version. It builds on COM and (among other things) provides the basis for the latest UI frameworks in Windows.
The overhead of socket read+write is typically much less than the serialization overhead, although both can be optimized to the point of irrelevance for many applications.
It's also interesting because it ends up looking like a microservices architecture, except all on one machine (even all in one process tree).