My knowledge is the 57 and I think these are identical regarding the autoflight system.
First of all is my opinion that flying the approach manually with autothrottle engaged is NOT a good procedure, either you do it all on the auto, or all manually.
Otherwise, you can easily get confused if something sneaks out of profile, -imagine trying to steer your car into the garage and your wife is stretching her leg controlling the gas....
In gusty conditions, the A/T is also sometimes a little bit too slow to follow the preset appr. speed.
Then pushing the GA switches, the autothrottle will anyhow "jump" in as long as A/T is armed on the MCP, but disengaged on the thrust levers.
There is no N1 reference on the 57, but during GA, you control the A/T by turning the bugspeed to command speed (VreF +80), you have to give her the speeds manually to follow during the GA, if you donīt, -she will try to keep you flying on the (low)speed existing at the moment you pushed the GA switches.
Then you could end up in a unpleasant situation...
regards, pete