Here in Massachusetts we use a standards-based test for 4th, 8th and 10th grade students (the grade levels tested is about to change). We've only had the results of one year to look at so far, but mony of the districts using Investigations were among the top scoring districts in the state. Of course, this is just a snapshot of student achievement, but the results have looked promising so far. I have also read many articles that I found from links on the CESAME web site which report that students using a inquiry-based program such as Investigations do very well on standardized tests. I've also read that students who have used both inquiry-based program along with traditional algorithm-driven instruction do the worst on such tests.
My district is concerned about student performance on these tests too, and we've decided to stick with Investigations. I'm convinced that the program really helps kids understand mathematics, and that should be reflected in the test scores.