That's actually a pretty good example of why this is a bad idea. You can't know which argument is being applied first unless you know the precedence levels.

In fact, according to the fixity you initially proposed, that translates to

(xs # map) $ (+3) . snd

The corrected version, xs # (map $ (+3) . snd), isn't much better. Even with the parentheses, it's needlessly confusing. ($) and (#) should never appear in the same expression, or at least they shouldn't apply arguments to the same function. It would be like using (.) and (>>>) together.