Isn't it a given that anyone with a mental impairment of any kind is less instrumentally rational [...] We don't usually give diagnosis to people who tend to win at stuff.
I am unconvinced by the broadest versions of the "rationality = winning" thesis, for reasons I've mentioned to alienist elsewhere in this thread. The very broadest version ("rationality = anything conducive to winning") would make, e.g., shortness or heart disease kinds of irrationality. A more reasonable intermediate version ("rationality = any features of one's cognition conducive to winning") still seems to me overbroad; it means that e.g. one person can be deemed more rational than another simply because ten years ago they happened to learn a particular language, or because they have a better (or worse) ear for music.
I think your observation that epilepsy would be deemed a variety of irrationality by the definition you're using is actually a handy reductio ad absurdum. Do you really want to define things so that epilepsy is a variety of irrationality?
For the avoidance of doubt: I will happily agree that, all else being equal, being autistic is generally a disadvantage, and that this disadvantage is a matter of cognitive deficiencies and not only of (e.g.) prejudice on the part of others. I just don't think "irrationality" is at all a good way to describe that disadvantage.
I propose a few ways of using these words.
"Instrumentally rational" = successful on account of being epistemically rational. Success for other reasons does not count.
"Epistemically rational" = correctly using observation and reasoning to arrive at true beliefs, and making the decisions and actions recommended by those beliefs.
"Irrational" = a privative concept, expressing not merely an absence of rationality, but an absence where presence is seen as having been possible.
How widely or narrowly you draw the line around "irr...
For example, what would be inappropriately off topic to post to LessWrong discussion about?
I couldn't find an answer in the FAQ. (Perhaps it'd be worth adding one.) The closest I could find was this:
However "rationality" can be interpreted broadly enough that rational discussion of anything would count, and my experience reading LW is compatible with this interpretation being applied by posters. Indeed my experience seems to suggest that practically everything is on topic; political discussion of certain sorts is frowned upon, but not due to being off topic. People often post about things far removed from the topics of interest. And some of these topics are very broad: it seems that a lot of material about self-improvement is acceptable, for instance.