For utility function maximisers, the AIXI is the theoretically best agent there is, more successful at reaching its goals (up to a finite constant) than any other agent (Hutter, 2005).

False. AIXI as defined can maximize only a sensory reward channel, not a utility function over an environmental model with a known ontology. As Dewey demonstrates, this problem is not easy to fix; AIXI can have utility functions over (functions of) sensory data, but its environment-predictors vary freely in ontology via Solomonoff induction, so it can't have a predefined utility function over the future of its environment without major rewriting.

AIXI is the optimal function-of-sense-data maximizer for Cartesian agents with unbounded computing power and access to a halting oracle, in a computable environment as separated from AIXI by the Cartesian boundary, given that your prior belief about the possible environments matches AIXI's Solomonoff prior.