First of all, thank you all for your responses. I've been busy with other things, and suddenly realized I needed to check this thread. I assumed I would be notified when other's responded. I see I need change my settings.
Alexsmom, thank you for the link. It confirms what other here are saying. Its nice to know even the pro-homework side realizes there is a point of diminishing returns.
Dude, your assessment of the school matches mine perfectly. Brilliant minds and all that.

As to why we have not moved him on to 6th grade, the answer is; its complicated. The local middle school, would be at best a poor choice, both academically, and socially. There are others in which he would be better suited, but they are magnet schools with waiting lists, and such. Since we are in a magnet school now, getting him in another, at the proper time, is easy, but not before then.
Also, he has been acting up at lunch, something he had never done before. Little things, mind you, but enough to let us know he is hard pressed at the moment emotionally.
And finally, we are due to have an evaluation (what they call an SST) on him soon, which could possibly turn into an IEP. Our DS appears to have dysgraphia, or at least he shows most of the symptoms of it. In addition he also is dealing with a growing inability to focus. I don't know if it is ADD lite, boredom, or something else. We got the opportunity to observe him last night at a meeting hosted by a nearby middle school with a crazy awesome focus on science (DS is WILD about science), and even when he was the most focused, he could not sit still. He was rubbing his face, his head, sliding down in his seat, sliding up. When I put my arm around him to steady him some, he started putting my hand on the top of his head, and holding it there. Things like that. We know what he is like at home, and just always assumed he was "that" way. Seeing him next to his peers in that context was illuminating.
Anyway, we knew something might be up, and while the magnet program is chock full of enrichment, it is also chock full of school psychologists, a school nurse (an unbelievable rarity in elementary schools around here) and the like. Meaning we're in a good place to support him if he needs to learn to take meds, or just do some behavioral changes. A middle school with multiple teachers and a wild pack of kids would likely be too tough on if he had to also deal with that stuff.
btw: love your handle. I'm a native Californian, and thus can say the word "Dude" 20 different ways, all with different meanings.