Im sure sony would have thought of this and if it wasnt supposed to work they wouldnt allow you to upgrade the FW. But honestly this is the first I have heard that you could use US FW on JP PS3. I must have missed something.
I dont think this would cause any problems because if u think about it the games are region free so if u have a jap system and decided to wait for motorstorm usa for the online play i am guessing it will come with the update on the disc so if it would damage the system sony would have made the games so they wasnt region free.
I am only guessing and i havent tried it, this may work as an advantage and take the censorship off the jap system ??