after a restriction one needs to apply boundary conditions and SYNC the grid functions. This SYNCS (as far as I know) the whole grid function. However for a restriction only a (hopefully small) number of grid points should be affected.
It may thus make sense if Carpet were to exchange only the data that needs to be exchanged.
This seems to be not quit trivial to implement though since one has to translate from the send region to the recv region in the sendrecvs.