If the new lens has got much better AF I might be willing to trade a bit of aperture ... but I'll be wating to see the Panasonic 100-300mm too (I think it's also f/4-5.6 ?)
So you are using 300mm f/5.6 on a 4/3s Body with In-body IS? How is it doing? I honestly can't believe a lens that slow (or even slower at f/6.7) is good for anything except neat motionless subjects at the brightest conditions. What shutter speed can you do without tripod and what ISO do you need usually? My experience has been that the E-P1s IS is good for 1/100s at 135mm focal length, so I wouldn't expect a lens without OIS to do any better than 1/300s at 300mm. You would be at ISO 1000 pretty often I reckon.I see that 75-300mm is f/4.8-6.7.
The 4/3 version (I have it) is f/4-5.6, and for the purposes I use it for (birding) I need all the aperture I can get ... on the other hand the 4/3 lens is agonizingly slow to AF, especially at the long end, which is also a problem!