complexity of modal logic axioms
You might find interesting some of the results in On the Complexity of Fragments of Modal Logics by Linh Anh Nguyen (in Advances in Modal Logic, 2004: 249-268). A version of the paper is available in the home page of Nyugen.

And probably you are already familiar with A guide to completeness and complexity of modal logics by Joseph Y. Halpern and Yoram Moses (in Artificial Intelligence 54, 1992, pp. 319-379). A version of the paper similar to the published version is available in postscript and pdf in the home page of Halpern.