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.