That's what happened to me many years ago when I had an SA Com/IR.
Went to FAA @ van Nuys in California and was given a commercial licence, endorsed as not valid for hire or reward. Went back a while later with a JAR and a SA ATPL and was told that I still had to write all the exams and take the check rides. If you google up and then telephone any FAA office at a relatively small US airport, such as Northeast Philadelphia,and then if you catch one of their good guys on the other end, you'll get chapter and verse and an invitation to do your check rides with him. Perhaps what this means is that the US doesn't do validations and if that's the case then the requirement for Federal Regulatory compliance can be no bad thing?