I'm like 60% sure that its not that article I had in mind, but the idea is the same (incremental increases in rationality don't necessarily lead to incremental increases in winning), so I feel pretty satisfied regardless. Thanks!
Could the article you had in mind be this?
In any case, Eliezer has touched on this point multiple times in the sequences, often as a side note in posts on other topics. (See for example in Why Our Kind Can't Cooperate.) It's an important point, regardless.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.