Without a doubt you will see major difference in sharpness, speed of the AF motors in the lens and overall better build and quality. I can tell you from experience, I have two 100-300mm lenses, there is no comparison between the new 100-400mm Leica and the old 100-300mm zoom.
Thank you for the answer. I'll continue to find a way to come up with some cash! I tried trading in the GX7 and 100-300 in two places. The GX8 and 100-400 seem to have pushed the older setup to very low price levels, so that didn't help much!