Here's what "A Glossary of Developmental Education Terms," JCRL (1991), 23, 2, pp 1-13 lists on the topic:
"Developmental -- l. a sub-discipline of the field of education concerned with improving the performance of students. 2. a field of research, teaching, and practice designed to improve academic performance. 3. a process utilizing principles of developmental theory to facilitate learning." p.4
"Remedial -- instruction designed to remove a student's deficiencies in basic entry or exit level skills at a prescribed level of proficiency in order to make him/her competitive with peers." p.9
Although the 33 "experts" who developed the glossary semantically identify the two terms in ways that would satisfy users who want to employ the terms both tactically and with professional/political "correctness," truth is the terms are virtually convertible except that one suggests need for "improvement" while the other suggests need for "remedy." My regional experience is that east of the Mississippi "developmental" is in fashion; west of the Mississippi, "remedial" has the usage edge. As long as ACT, ETS, College Board, League for Innovation, ERIC, Clifford Adelman (U.S. Department of Education), and, intermittently, John Roueche (U. of Texas) continue to use "remedial" without apology, perhaps so might we.