int2 vs int4 in Postgres

Is there an performance benefit to using int2 (instead of int4) in cases
where i know i will be well within its numeric range? I want to conserve
storage space and gain speed anywhere i can, but i know some apps simply end
up casting 2byte data to 4byte (like Java int/short).

These int2 values will be used in primary and foreign key fields and I know
that i must explicitly use tick marks (ex: where int2_column = '12') in
order to make use of indexes, but my question is IS IT WORTH IT?  IS THERE
ANY REAL GAIN FOR DOING THIS?

An simple scenario would be:

Songs
-------
song_id serial pkey
genre   int2   fkey
title    varchar
...


Genres
-------
genreid int2   pkey
name      varchar
description varchar



I KNOW that I am not going to have anywhere near 32,000+ different genres in
my genre table so why use int4?  Would that squeeze a few more milliseconds
of performance out of a LARGE song table query with a genre lookup?


Thanks,

-Aaron