change of QNH during approach.
 

if QNH change during approach, do the FMGC recalculate app speed?
is the new QNH changed in the FMCG?

with practical experience i can let u know tht...
the speed is recalculated only when the value of the qnh is changed in the perf page of the fmgs
hope this helps

QNH change on the PERF APP/vertical revision@destination does NOT re-calculate FM predictions. In fact QNH is not taken into account for the FM descent profile, check it out whenever in DES mode and when cleared to an altitude/setting QNH: the FM profile will simply re-adjust itself to be ON profile after you've bumped the indicated altitude up or down.
A QNH change on approach would only be selected on the EFISCP. The 'purists' out there may elect to also change the QNH in the FMGS, the more elegant way would be to do that by means of a lateral revision at the destination on the FPLN page (PNF).