Hello I tried opening the US first level boundary file in Google Earth, however it loads exceedingly slowly, so much so as to be impractical. This happens whether I open Google Earth in DirectX or OpenGL mode. Perhaps you should try making less detailed files for Google Earth. My version is 5.1.3533.1731 and I am using Windows XP Service Pack 3.
file size
You are right, the files have a very high precision and that makes processing them slow for larger countries. In the future we would like to provide the data at different resolutions.