Good idea. I achieved it this way in the example file: Duplicated the group prototype and set its width to e.g. 640 (iPhone 5). Created object from the new prototype and moved original group content into the new one. Now any change in Cell and Section header prototypes will be reflected on both iPhone 6 and iPhone 5 artboards. Pretty cool.
Here’s the amended example file: http://cl.ly/1B0m0E1V3g16
I also tried applying flexDirection:column; alignItems:stretch; on the original group prototype, which should eliminate the need of separate group prototypes, but for some reason it doesn’t do anything. Might be worth further investigation.