I actually tried it today. Couldn't find anything about it in the FCOM but on the V2500 engines, selecting SPEED in stead of MACH actually gives a faster reaction in A/THR operation at high altitudes.
ALT CRZ still allows for the +/- 50 ft, but A/THR reaction to speed changes are faster. Interesting to see but nothing to backup this "feature" in the FCOM
Anyone any reference on this??