I, too, have experience with the SAT10 and not the TerraNova. I've administered the SAT10 to homeschooled students. The tester tells the student that s/he has about x amount of time to complete each segment but it isn't technically timed and the student can go over the time allotted with no issue. The tester is encouraged to move the testing along and encourage the child to finish if the child is taking forever and is clearly not being productive.
The ITBS has strict time guidelines. If the child isn't done in x minutes, the test is stopped and that segment left incomplete with the undone questions marked incorrect. I've generally recommended the SAT10 over the ITBS to parents whose kids are slower workers for the reason. That doesn't sound like your dc's issue, though.
The thing that was interesting to me on the SAT10, though, was that it actually seemed to net some pretty high percentiles in the upper elementary grades even when a good number of questions are missed. For instance, in the 5th grade version, I've seen a child miss about 25% of the questions on science and social studies and come out around the 90th percentile on both of those tests. The language arts tests seem to be less forgiving with just a couple missed questions bringing the total down to around the same spot (90th percentile or so).
I've never tested a child who has done unusually well on the math portions so I wouldn't know how high the head room is on that part. There are two math subtests. One is straight calculation. The second is more of a reasoning test with a lot of word problems.
