Grails: UI-performance plugin
With everything bundled and compressed the size of the the page download is reduced from 485 Kb to 187 Kb.
I only had some minor configuration problems but those were just related to my own typos. Apart from that it all now works perfectly.
The plugin saves me the time it would take me to produce a build step to produce the same output, and it even does more than the initial idea for my own build step. Thank you Burt Beckwith for a job well done!