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?
These should help you:
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.
Perceived loudness can be calculated as

k^(1/3.5) where k is a ratio of power (or squares of voltage)

in your case (0.5^2/0.3^2)^(1/3.5) =1.34

0.5mV will be 34% louder than 0.3mV

Another typical example 200W amp will be louder than 100W amp by (200W/100W)^(1/3.5)=1.219 times (21.9%)
I own two cartridges that have outputs of .3mv (Transfiguration Orpheus L) and .5mv (Lyra Titan). While it is impossible to convey my subjective impression of the difference, I will say that the Lyra is louder, but, not by as substantial a margin as I was expecting.
When comparing the output of any two cartrdges, make sure both cartridge manufacturers used the same test track and measurement protocol. Not all of them do, so it's easy (and pointless) to compare apples with oranges.