Hi,
If I was you and wanted to go to the US for some hour building, I should go for FAA PPL and then FAA Instrument Rating. I think its well spent money to go for some training even if you have your JAA PPL already, ie. training emergencies(you can never do too much of that!) and some confined areas and stuff like that you can use later when going for your JAA CPL. Also if you do your FAA Instrument Rating its training that you can be credited for when doing your JAA IR....(saving alot of £)
I was training with Vortex Helicopters in MS some years ago and was very happy with the training. The airspace is not to busy and Joe (the owner) is a very nice guy that will help you out.
When you go for your training, remember to go for QUALITY training, because that are the basic fundamental skills and attitudes you are going to use the rest of you career
Good Luck