If you go the FAA route, you will also need to pass the FAA CPL as well as the FAA ATP, don't forget the new ATP compulsory ground school as well.
You qualify for the FAA if you hold a foreign commercial with an IR. But you have to have your CAA verify it to the FAA. The ATP CTP takes about a week but doesn't cover much of the actual test subjects. So add another few days of self studying. As pointed out the single written isn't much but the oral exam can go on for hours with no multiple choice answers provided.