Eh sorry, aber hast du die 25600 mal mit der Konkurrenz verglichen? Es rauscht weniger und zeigt zusätzlich mehr Details. Das liegt nicht an der JPEG Engine sondern daran dass der Sensor einfach weniger rauscht.
(auch in den RAWs gut sichtbar, ebenso in den niedrigen ISOs)
Entscheidender ist was bei unteren Isos passiert, 25600 ist sowieso unbrauchbar, auch wenn sie besser ist.
http://www.ephotozine.com/article/panasonic-lumix-dmc-gx7-expert-review-22581
"compared to the Olympus OM-D E-M5, the GX7 images at ISO125 and ISO200 show smoother results, with slightly less grain visible, although the E-M5 shows more fine detail. Noise results at ISO400, ISO800, and ISO1600 are very similar, however with the E-M5 retaining slightly more fine detail. This is the case at ISO3200, where noise results are very similar, although there is a slightly higher level of noise in mid greys from the GX7. Again at ISO6400 results are very similar, with both cameras smudging detail. At ISO12800 image quality from the E-M5 deteriorates, and at ISO25600 the images from the GX7 show better results, although the highest ISO setting on both cameras is best avoided if at all possible."