Hello
Please take a look at this post: Need a Jumpstart with Delphi XE3 and .NET 2.0 DLL Assemblies Integration
It shows how to provide a Plugin-Host interface.
There IS a performance hit, but it is not he plugin or host performance issue. The performance is affected by the process of calling a .NET method from unmanaged code as it involves parameter marshaling etc. Still this performance hit is visible only if you are going to call some plugin method from host thousands of times per second.
Regards