In my opinion, nothing is really wrong with the Panasonic 8mm in a zen dome port. But leaving aside financial constraints, most of us always are looking for newest and bestest :-)
Beyond that, I do think that GENERALLY, the Oly lenses on Oly bodies seem to produce less ca and I sometimes cannot completely get rid of all of it in Lightroom. Also, being a 1.8 gives a bit more latitude on shutter speed which can sometimes be nice, and may help with faster focusing. Additionally, it seems to me that the Panasonic gives best results around 5.6 to 6.3. Faster than that and I think the edges start softening more, but closed down more the images in general seem a bit softer, perhaps due to diffraction. Since most lenses are at their best 1-2 stops from max, a 1.8 MIGHT give a wider range of apertures that yield best results.
In the end, these differences are minor and my skills, not the camera and lenses, are the major limiting factor in my photography. Still, I am eager for the 7-14 and the 8mm to arrive.
In the meantime, I enjoy the Panasonic the lens quite a bit.