Test Outcome:

• Checking if a variable is null with "if(myObject)" is about 10x more costly than doing "if(System.Object.ReferenceEquals(myObject, null))". The ReferenceEquals method is even faster than checking if a bool is true/false.
• A successful cast is more expensive than an unsuccessful cast
• Casting is a relatively light operation. It is about as light as checking "if(myObject == null)". But this might vary with the size of the object (?)

The Test:

The following test:

    public bool testBool1 = false;
    public bool testBool2 = true;
    public Collider testSphereCollider;
    public Collider testBoxCollider;
    public Collider testNullCollider;

    public void Awake()
    {
        Stopwatch watch = new Stopwatch();

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testBool1 == testBool2)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Comparing bools " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testSphereCollider)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Checking null on testSphereCollider " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testNullCollider)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Checking null on testNullCollider " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (System.Object.ReferenceEquals(testSphereCollider, null))
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Checking null on testSphereCollider with ReferenceEquals " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (System.Object.ReferenceEquals(testNullCollider, null))
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Checking null on testNullCollider with ReferenceEquals " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testSphereCollider as SphereCollider)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Casting testSphereCollider to SphereCollider " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testBoxCollider as SphereCollider)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Casting testBoxCollider to SphereCollider " + watch.ElapsedMilliseconds.ToString());

        watch.Reset();
        watch.Start();
        for (int i = 0; i < 10000000; i++)
        {
            if (testNullCollider as SphereCollider)
            { }
        }
        watch.Stop();
        UnityEngine.Debug.Log("Casting testNullCollider to SphereCollider " + watch.ElapsedMilliseconds.ToString());
    }

Outputs this: