Calling a generic method with interface instances

As a followup question to this one

public interface IFeature  {  }

public class FeatureA : IFeature { }

IFeature a = new FeatureA();
Activate(a);

private static void Activate<TFeature>(TFeature featureDefinition) where TFeature : IFeature
{

}

I undestand, that once the FeatureA is casted to IFeature the generic method will always get IFeature as type parameter.

We have a service with provides us with a list features (List<IFeature>). If we want to iterate over those features, passing each in the generic method, I guess there is no way to get the concrete type in the generic method other than

• using reflection
• using a dynamic variable to determine the type on runtime (Calling a generic method with the correct derived type)

Since reflection is very costly, I would like to use the dynamic cast. Is there any downside to call the method that way? Somehow I feel dirty when doing that :-)