I don't think the problem is the curriculum, or the number of days we go to school. Rather, I believe it is that there is a lack of motivation among American parents.
In Asian countries, parents pressure their kids to do well, as getting bad grades is considered dishonor to their family. In America, however:
"Mommy, I got an A on my test."
"OK."
and
"Mommy, I got a C- on my test."
"OK. Just don't fail."
Most parents these days don't care if there child gets Cs and Ds. (Well, in America) Also, kids are also pressured by their peers to not be "a nerd", or else they will get bullied.
I can understand a week or so trim off of summer break. However, getting rid of summer and weekend breaks is overkill, especially since the main issue has nothing to do with the number of days a child is in school. Also, as a few others have said, this cuts in to overall family time, which actually worsens the issue of parent motivation. Not only this, but the extra hours for the gifted students are a waste. Even now, I get at least an hour of "free time" during the school day. I couldn't imagine going to 6 PM, and going on a Saturday.
Having school on Sunday also interferes with religious activities of some students. This can be a big punch in the face to some families, and will cause them to give up public school altogether.
Also, going for SO many days on end can cause burn out. Going for 60 hours a week (10 hours X 6 days) is more than the amount of hours the average adult works. Few jobs can match 60 hours a week, and you're placing all of this on a child. Work-weeks like this even cause stress for adults, let alone an 8-yr. old.
Overall, if this is implemented, I see it failing miserably. I don't even see many parents approving of this, due to the cut in family time. Burn-out is also likely to happen among students, and I can see wide-spread skippings.
Moves like this will make sure you won't get re-elected in 2012, Obama. NCLB was bad enough.