[Frédéric Amella]"Does transfering ex1R with log and transfer on FCP7 made a new encoding of the video and then make a lower quality ?"
No. it doesn't make a new encoding.
[Frédéric Amella]"When i use log and transfer from EX1R the clips stays in XDCAm HD (it is the same compression as the native clips ?)"
Yes it's the same compression. Both XDCAM Transfer Tool and Log and Capture Tool work the same way. All they do is use the same video clip and re-wrap it in a different "container" that Quicktime and FCP understand. To over-simplify, it's the same present in a different box and wrapping paper. FCP likes blue paper and Avid likes grey (I think). :-)