Get the exerpt for a given post id after filters applied

Using get_var I can easily get a post excerpt. I can also get it using get_the_excerpt(), but that one returns the content if there’s no excerpt (which I don’t want). But more importantly, neither of these gives me the excerpt after filters for any plugins are applied.

I need to get a post excerpt (not necessarily the excerpt for the current post) after all relevant filters have been applied. Any ideas?