The detail of the 14-140mm lens drops off from 100mm-140mm, any lens with a large focal range always makes compromises in quality at either end of their range.
The image quality across the focal length range of both lenses (the 14-140 and 100-300) is very close (does not vary noticeably), although there was some deterioration in the longer range from one of the 14-140s I checked - but not anywhere near enough to account for the difference I see here. What is not close is the image quality from the 14-140 (and I have checked three of them) compared to the image quality from the 100-300s. At 300mm the 100-300 is essentially the same as what you see here, where the worst aperture of the 100-300 (and one far faster than the 14-140) is quite a bit better than the best aperture (almost two stops slower) of the 14-140. I tended to use mostly primes until I switched the the Nikon 3D and 3Ds, but none of the zoom lenses I have (Canon 70-200 f2.8IS, Nikon 27-70 f2.8, 70-200 f2.8VR II or 200-400 f4.0) exhibit any noticeable falloff at their maximum focal lengths. We really do need a better lens, and one that is at least f4.0 at the maximum focal length. If you are working in a way that allows for the use of other lenses, there are options, but for following fast motion, AF is almost mandatory on the GH2.