It will be louder, but it will create a 66% increase in apparent loudness. Take a look here for an explanation, which may help you with the Vinyl Engine's calculators.
Cartridge Output
How much difference is there between the output of a .3mV cartridge and a .5mV; is there a way to compute the approximate change in volume between the two? My dynavector 20xl is only .3mV and when used in my Magnepan rig I often wish that I had another 5db or so of volume at the listening position. Will moving to a .5mV cartridge increase the volume level?