Fastest Convert from Collection to List<T>

What version of the framework? With 3.5 you could presumably use:

List<ManagementObject> managementList = managementObjects.Cast<ManagementObject>().ToList();

(edited to remove simpler version; I checked and ManagementObjectCollection only implements the non-generic IEnumerable form)

You could use

using System.Linq;

That will give you a ToList<> extension method for ICollection<>

managementObjects.Cast<ManagementBaseObject>().ToList(); is a good choice.

You could improve performance by pre-initialising the list capacity:

    public static class Helpers
    {
        public static List<T> CollectionToList<T>(this System.Collections.ICollection other)
        {
            var output = new List<T>(other.Count);

            output.AddRange(other.Cast<T>());

            return output;
        }
    }