The theory of below -40 is that all the moisture should be ice crystals and as such should pass through a fast rotating fan without adhering to the surface.
When in the decent not only are the stall margins of the engines much smaller but the fan is rotating much slower allowing ice to build up on the surface of the fan.
Its not just a 320 issue, we also did it on the 757.
As for the speed brake, all I know is you get half speed brake with the AP engaged on a 320 and full on a 321. Why? I have no idea!