Potting probably reduces the diffusion of electrolyte, but I wouldn't count on it to help significantly, you're stopping rubber with rubber.
Also, that electrolyte has to go somewhere. Most of it will remain diffused inside the potting, but some of it may condense in pores and interfaces in and around the potting, which could be an electrolysis/corrosion hazard just the same as any leaky cap is (but more limited, to such locations).
Conductivity: air sucks, particularly when the device is compact and there is little room for airflow within the enclosure. Typical figure is 0.045 W/mK, but this is going to be a hand-wave because heat diffuses rather than conducts, and some convection always occurs.
It can be much higher where convection (chimney effect) takes place, but this requires more room, or more temperature differential.
Speaking of, temperature differential. Convection only requires that, and gravity. Putting some components inside a closed metal box incurs two convection steps, so the peak internal temp rise is almost double (very roughly speaking!), compared to the case for an open or well-ventilated enclosure.
A fan can be desirable inside a sealed enclosure, for this reason. Better is just to design it so everything sinks heat to the enclosure directly.
These may be of interest,
http://www.cde.com/resources/catalogs/MLP.pdfbut good luck budgeting for it
Tim