Currently, /ws/2/recording?query= returns a <release-group /> element per <release />, but it only has a type attribute:
<release-group type="Album"/> (http://musicbrainz.org/ws/2/recording?limit=10&query=%22Wattershed%22%20AND%20artist:%22Foo%20Fighters%22%20AND%20release:%22Foo%20Fighters%22).
To perform a lookup of release groups, you first have to fetch the releases to get the release group IDs, which adds a lot of time to the operation (given that you can do at most one connection per second). Adding the ID to the already existing element would save a lot of time.
In addition, since I need release groups to get the original release date, it would be even better if <first-release-date /> could be added to the those results (I need it to rewrite Amarok tagger, and I've already - unsuccessfully - explored other possibilities with some developers in #musicbrainz-devel).