So without the dummy load, you get 225 watts per channel and each cab gets 225 watts. With the dummy load, the amp now puts out 320 watts per channel. However, of that 320 watts per channel, the dummy load gets half (160 watts) leaving 160 watts for the cab. You've dropped your cab from 225 watts to 160 watts.
Meanwhile, each dummy load is pumping 160 watts into the room, better make sure they are fan-cooled.