There is a well known theory in psychology called Cognitive Styles.
This theory was inspired by an American psychologist Herman Witkin and is based on a series of empirically proven scales discovered by several researchers.
For me, at least some of these scales seem much similar to the socionic ones:
Field-independence vs. field-dependence (Witkin) = logic vs. ethic (Socionics) or thinking vs. feeling (MBTI).
Abstract vs. concrete level of conceptualization (Harvey, Hunt, Schroder) = intuition vs. sensing (Socionics and MBTI).
The most important thing is that Jungian scales have always been criticized for lack of empirical data while the cognitive styles are empirically proven. Moreover, while socionics and MBTI prefer subjective methods of testing, such as questionnaires, interviewing, watching etc., the cognitive styles are based on objective methods of research, such as measuring the speed of reaction etc.
I believe that knowledge of the cognitive styles theory can contribute in further development of socionic diagnostics.