I got an 800 on my English achievement test and have worked as a professional editor. I couldn't diagram a sentence to save my life, and my knowledge of geeky technical grammar (as in, the actual technical terms) is relatively weak. I don't think bright, verbal children need a great deal of instruction in this, though some simple reminder rules (for instance, there are some easy cheats for remembering when to use "who" and "whom," and so on) are of use. Reading widely and, more importantly, being edited ruthlessly by someone who knows what they are doing will do a great deal more to improve writing skills.
Gifted kids who are very math-heavy may be a different matter. However, even in that case, I'm not convinced that treating grammar like math instruction or like a puzzle to be solved is terribly effective--I think it's more holistic.
My DD seems to be much like me and is picking up beautiful grammar and sentence construction by osmosis. In fact, she notices errors in books and points them out to me. Be still my heart!
