cdfox, I have to respectfully disagree with the idea that practice (at the appropriate level, as Karl has already pointed out is important) will harm or fail to help your son progress in math. It's likely that one big reason your son needed no practice on pre-K to 1st grade concepts is that he already had a good grasp of them; it's unlikely, though I suppose not impossible, that he would be able to learn math through a postgraduate level instantly at a glance. I would agree that lack of motivation and boredom can be major problems, and I think that's all you've seen. A person cannot unlearn concepts through extra practice; they can only become demotivated.
The way I see it, there is a point of diminishing returns with extra math practice. Benefits of math work in general include learning or refining new math concepts, and learning or refining skills such as computation algorithms or techniques. Learning a new concept as an overview does not necessarily mean that the concept is mastered completely. Being able to demonstrate knowledge of a computation skill doesn't mean that it's mastered either; practice can improve accuracy, speed, etc. Limitations on benefits of math work would include decreasing improvement in accuracy and speed, decreasing improvement in conceptual knowledge by further work at the same conceptual level, and decreasing student involvement due to boredom.
It's obviously true that children with a math bent can learn math concepts more quickly than normal. All I'm saying is that this doesn't mean that they fall off in ability or understanding with more work in general-- it just has to be appropriate work.
I also recall finding some of Linda Silverman's work questionable, for instance her widely repeated assertions about parents being excellent identifiers of giftedness. I'd have to read up on her work with visual spatial learners to form an opinion of what she's said about math practice. I'd be grateful for a link to the research you mentioned, or else I can try to request it through the interlibrary interspouse loan system.
