The conceptualization of the followers' developmental level is not especially clear in situational leadership. This makes testing the theory difficult.
Here are some studies which have examined situational leadership, generally only finding weak evidence supporting the theory:
Fernandez, C. F., & Vecchio, R. P. (1997). Situational leadership theory revisited: A test of an across-jobs perspective. The Leadership Quarterly, 8(1), 67-84.
Graeff, C. L. (1983). The situational leadership theory: A critical view. Academy of management review, 8(2), 285-291.
Vecchio, R. P., Bullis, R. C., & Brazil, D. M. (2006). The Utility of Situational Leadership Theory A Replication in a Military Setting. Small Group Research, 37(5), 407-424.