its gunzipping process that eats time and there's no way to reach the tar archive members before gunzipping. I don't know how much will help to explicitly specify a member offset when listing the tar but if the member lookup process is faster than fully listing then:
tar -K etc/passwd -tzf backup_2010-09-27.tar.gz| head -1
i doubt it tho.
you can also try to extract the file and check the return code so you will get rid of fully listing the archive and grepping afterwise.
echo etc/passwd | tar -T - -K etc/passwd -Oxzf backup_2010-09-27.tar.gz >/dev/null
$? should be zero
still, gunzipping ....