Just a little addition to checkboard's response - all the stuff about Conc engines true except they are at their MOST efficient at M2.0. There are huge benefits in pressure recovery attained by decelerating the flow from M2.0 to M0.4 (in 10 feet!). Like a supercharger upstream of the LP compressor.
MPG at M2.0 compares very favourably with any other heavy jet in CRZ.