Just some quick thoughts.
I think "enough" is different for every kid. First and foremost, is kid happy most of the time? If no, you have a problem. If yes, then you have to decide about the academics being "enough."
Here's where the crystal ball comes in. Some kids would do just fine w/ enough. The man who wrote Art of Problem Solving materials went to an avg highschool w/ about 30min of busywork homework each night. He said he didn't learn much. He said it was the best thing for him b/c it gave him FREE TIMe to pursue things he wanted to pursue. Now that's at the highschool level and quite different from elementary.
My point being some kids will be just fine. You challenge them at home since he won't have a lot of homework, on summers and weekends. Follow his passions.
Other kids will wither away as mine did. His whole personality changed. Plus, he was having 2hrs of homework each evening and we had NO time for afterschooling his interests. And he was in such a foul mood from school, there was no point.
Do I think it will get more challenging? Probably not. I think if you're dealing with an academically motivated child, a bright child, perhaps an MG kid in a good school - then yes. Once everyone learns to read and gets beyond simple adding/sub/multi/div then things will get more interesting. If you're dealing w/ an HG+ kid, probably not. If you're already 2-3+ years advanced (even if not in material but in acquisition of new info, making connections), then another year will not make a difference. That was my son's issue. He had things he needed to be learn, to be exposed to .... but they entire school year could have been compressed into about 4-5months.
And you mention if it's like sports.....the other kids might have spurts and have better understanding....BUT, your son will also have a spurt so you see, the distance is not closing and in fact might be getting wider.
You really have to look at your DS's personality. It might be enough for him. My Dh said "but in high school he'll have AP classes." the only problem was that DS was in 2nd grade.....that's a long time to wait to be challenged. BY then, your brain is mush, you don't see school as a place to learn, you're hanging out w/ the druggies at recess....
I think b/c most adults want the easy way out and don't want to be challenge (we have many items competing for our attention) that they can't imagine that kids thrive on challenge. People think it's great to be smart and just be able to coast. And for some, that is certainly true....gifted underachievers. But for many, lack of challenge is detrimental.
