Certainly the benefits to acceleration are obvious. Institutions across the country often see a loss of 10%-20% of students initially identified as needing 2-3 remedial courses. They simply never enroll in a remedial course. Among those students who do enroll and succeed in a course, around 10% fail to enroll in the next course in the sequence. In the most recent 6 year cohort (students who started in 2009) of state level data on community colleges in California, over 60% of students who were initially placed two levels below college-level math failed to complete their transfer-level math because they failed to enroll at some point - not because they failed a course. Interviews with students suggest a variety of causes. Many community colleges students face other life challenges that make getting to school hard. Anything that stretches out the process makes the successful completion more difficult.
We also know that placement tests (particularly in settings that don’t sufficiently communicate the stakes associated with the assessment and don’t encourage student to study and practice beforehand) tend to place many students in courses lower than their actual ability. Several studies suggest that this under placement is particularly likely with students of color. Such placement policies exacerbate gaps in completion given the problems described above.
The work done in Tennessee and data made available from their statewide implementation suggests that corequisites (specifically when students are placed in the appropriate college-level math pathway course and given additional supports in keeping with their needs) can work for many students to produce a successful grade in the course. What this data also makes clear is that there are a substantial number of students for whom such a solution does not work. If colleges are able to accurately identify students unlikely to succeed in a corequisite setting, longer cohort models or other more intensive designs might have more success.
What we still do not know is how well students prepared through corequisite math courses perform in other general education or in upper division courses that require quantitative reasoning. As noted in the first paragraph, acceleration is likely to deliver a substantial improvement in success rates even if it delivers nothing in terms of improved math understanding. There are corequisite approaches that combine a traditional math course with a support course. There are other approaches that provide students with a substantially redesigned course designed to improve the math conceptual knowledge for at-risk students. Both will yield higher success rates than traditional remediation sequences. The former reflects business as usual, the latter requires substantial changes in material, classroom management, and pedagogy. There is no ready research that tells us if those differences matter.