I doubt further testing would make much difference towards a decision to put her into early K, she's far enough ahead as it is, and with her later Birthday I suspect the schools would be concerned from the social side. Also, by the sound of it K will probably be almost as basic to her as pre-school - still CVC words, basic adding etc. The one area that might make a difference is how good her fine motor/handwriting is yet. Would pre-school be a whole day thing or half? Being bored for half a day is better than a whole one! If you are with her in the afternoons you would still have the opportunity to show her the world, provide encouragement, and develop her interests in all sorts of areas. In my book this would be more beneficial than full day early K.
Of course, this doesn't help with when she finally does have to go to school, but then a Grade skip or subject acceleration may be easier.
