Major et al (2005) is interesting. The bottom line for my ideas is that "ESL listeners performed notably better on tests with lectures delivered in Standard American English and Southern American English than on tests with lectures delivered in ethnic or and international dialects." The ethnic and international dialects were AAVE, Australian and Indian. (Australian is used in IELTS).
There is a danger of being swamped by the overlapping aspects of this study: P&P, testing theory, ICC and CDA...
There's another point that came to mind when reading this article: the assumption that teachers having non-RP accents is A Good Thing. It might be, but whence the paradigm? Could it just be an example of what a succesful old school teacher described to me as touchy-feely-tefly-wanky, (or TFTW)?
So I need to look at my basic methodology texts for that, and also some of the refs from the Major article.
Major, R. C., Fitzmaurice, S. M., Bunta, F., & Balasubramanian, C. (2005). Testing the
effects of regional, ethnic, and international dialects of English on listening comprehension.
Language Learning, 55, 37-69.