Nice Kata :-)

The only suggestion I have is to write the tests using the TestCaseAttribute. Rather than breaking on the first failed test, it runs all the tests and you can clearly see which ones fail.

It will look like this:

  [TestCase("AZ", ExpectedResult = "A                         Z")]
  [TestCase("ABC", ExpectedResult = "A B C")]
  [TestCase("ACE", ExpectedResult = "A  C  E")]
  [TestCase("CBA", ExpectedResult = "  A")]
  [TestCase("HELLOWORLD", ExpectedResult = "     E H    DLL   OLO   R  W")]
  public string BasicTests(string input)
  {
      return Kata.Speedify(input);
  }