The main advantages of the m4/3 system are size and weight but the new Olympus 300mm will have to be quite sizeable as the F4 aperture will need to be 75mm in diameter and as one of their 'Pro' series lenses they won't be cutting many corners on the design and construction. So the full-on E-M1 + battery grip + 300mm + 1.4tc will not be that much smaller than some DSLR set-ups. However, any additional lenses that you may require can pull back the advantage - lenses like the 25mm and 45mm F1.8 designs are amazingly small and light for their specs.
Of course, until the 300mm is released we won't know just how heavy it will be - and how expensive! - but to make up some way for the smaller sensor size it will certainly need to perform extremely well at full aperture so as to enable the lowest ISO setting possible.
One wonders whether Panasonic may be working on a more compact F5.6ish 'Leica'-branded alternative design for those wanting to stick to the small/light m4/3rds concept...