Thanks for all your advice.
I'm interested in getting a better and safer private pilot. That's why I'm thinking to go for the IR. I had some instrument (JAA) dual time and regularly fly an N-registered aircraft here in Italy.
As I can't take 1 month off from work for the IR training I need a "fast-track" syllabus to be done in 10-14 days. The knowledge test will be done in the UK before going to the US.
I'm not fixed on a part 61 school, and I'm not trying to "trick" the system, going on the visa waiver program, but nevertheless I'm looking for the official informations and it seems not easy to find it in writing, just opinions.
I'll be very thankfull to everybody who could help me.