I have tutored the SAT for over 15 years. Countless kids ranging from learning disabled to HG+. Here's my two cents:
Preparing for the test will help. Some more than others. I can usually get a child up 100 points in math (if they are starting below 650.) I get plenty of kids who can score a 750 on any given day but want an 800 (why is a different issue.) A good tutor can usually get them there too. The preparation for those kids has mostly to do with training them to avoid careless errors and identify in advance their own tendencies when it comes to careless errors. But a gifted child typically has no problem internalizing those lessons quickly.
Reading is tough to budge. Maybe 50 points if you are very lucky, but the margin of error is about that, so it is tough to say.
Writing is surprisingly easy to move, again, if you are at a low enough starting point. Grammar isn't taught in school much and the multiple choice portion is very grammar heavy. The essay is graded on a rubric and is pretty formulaic. You might not be able to get someone from an 11 to a 12 (perfect), but you can get most kids up a point or two if they are clearly doing something wrong.
Timing on the test is rarely a major factor. A typical MG or HG kid has time to spare. And because of the way it is scored, where you lose points for wrong answers (as opposed to skipped questions, which result in neither adding nor losing points), the timing often benefits lower performing kids because they don't have time to get to the questions they would get wrong anyway. With all due respect to the prior posters, if a gifted child has issues with the timing, then this is actually an indication that they will, in fact, have trouble at the college level with timed tests. I suppose you could find your way through some college program without timed testing, but it would be difficult. In this sense the timed nature of the SAT is an important element in determining a student's aptitude for college level work, which is of course the primary purpose of the test.
The math portion of the SAT, much more than the ACT, does an excellent job of differentiating those children who do well at school because they are good kids who work hard and do what they are told from those kids who have a natural aptitude for mathematics. The math is much more about creative problem solving then it is about math content. In fact the average student is done learning the content they need by the end of 10th grade. And the computation is minimal, even though you are allowed to use a calculator. Most of my gifted kids won't even use it.
As mentioned in prior posts, the reading section can definitely be an issue for gifted children, because of their ability to see nuance in the questions and answers. This is not an issue in the writing portion of the test.
I think it is worth remembering also the target audience for the SAT (a 17-year old getting ready for college) and the point of the test (assessing aptitude for college-level work.) A child at that age should have the maturity to understand why they are taking the test and the goal -- getting as high a score as possible. Unless there is an underlying issue, a 17-year old with a high aptitude for college level work will be able to modify their style in order to complete the task successfully -- this is what we have to do in real life! If they cannot, that might not be a statement about their abstract intelligence, but it very well does say something about their ability to adapt to a demanding college-level environment as an independent adult.
This is not to say the SAT is the end all, be all. It unfair in all sorts of ways, starting but by no means ending with the ability to tutor to it. But it is just a small piece of a college application, after all. And for the gifted crew, once you go above 700 in a subject area, scoring higher really makes no difference.
