There is a general norm of almost all super-zoom lenses, the IQ at wide to mid range would always be better (much) than that of the tele-end.
Albeit a 24-600mm F2.8/5.6 super zoom is very attractive/versatile, experience tells us that the tele-end was usually for emergency only. And the IQ would be at next-to-useless level (if we were a bit picky). However, if the final production unit could deliver very usable IQ at the tele-end, the G3X would be a very competitive product in the market.
In addition, I don't think the lag of 4K video capability would be a matter of concern ... anyway, 4K video is not everyone's cup of tea. However, the so-called "5-axis stabilization" ... is a bit tricky !