The 1% figure is taught on the Airbus Perf Course in Toulouse, but I don't have a direct reference.
The graph for an A340-314 at Fl330 (don't have the mass) gives a max SR of 62.1 NM per tonne of fuel at .783 mach, and a LRC SR of 61.5 NM per tonne at .803 mach. A significant improvement in speed for a small decrement in SR.