I don't think it's a constant 200.I'm not sure I would complain if they did, but I also don't think they really need to. The GH4 currently offers 200 Mb/s for 1080p, so SDXC should be fine for that bitrate...
If they can get a Back Side Illuminated (BSI) sensor, they might get 1-2 stops more, making it ~ 13-14 in photo Raw and close to it in V-Log. The problem is that Panasonic isn't making BSI sensors. Sony does but it hasn't sold it to Panny. Samsung does too but it seems it wants to dance with Nikon only. There are other manufacturers, obviously, and the GH engineers would have to find the right partner if their corporate superiors want to take a step forward. Unfortuinately, with the APS-C cameras going for as little as $600, the superiors may think MFT is no longer a pro format.I expect the GH5 will have similar low-light performance to its predecessors. It's the limitations of a m43-sized chip.
PS. As to processors, the major fabs have switched to the 14 nanoMeter technology in late 2014/early-mid 2015 from the 28 nM of the previous generation, making the internal HEVC processing on the sub-$2,500 cameras possible without the heat related problems.

