As mentioned previously on MotoringFile the 2005 JCW Cooper S kits should see a 10-20hp increase for relatively the same price. And according to sources those with the '04 or older kits you should be able to purchase an upgrade package that bumps your HP to that of the new levels as well. All this should be out as early as September.
However there is one downside to all this. The current 2005 MINIs that are going to be delivered between now and then apparently can only be fitted with the 2005 kit and not the 2004 kit which is currently available. That means if you get your 2005 MCS or MC next week you'll have to wait at least a month and probably longer before you get your JCW installed. For most the wait won't present too much of an issue but at the very least it's something to keep in mind.
You can read the official MINIUSA PDF here.