AIXI was neither invented by MIRI nor have they proved any nontrivial theorems about it.
AIXI is Marcus Hutter's work: http://www.hutter1.net/ai/aixigentle.htm
From a superficial inspection, Hutter's back catalogue features 89 * @inproceedings, 3 * @book, 12 * @techreport, 51 * @article, and 1 * @compression prize
Over an elapsed period of '87 (from before he had completed his masters degree) to the present, on a published_artefacts/year basis, Hutter is more productive than MIRI.