While not a perfect solution, grabbing screens at iPhone 6 Plus resolution and then down-scaling for iPhone 6 & iPhone 5 screen sizes is a lot quicker.
To do this I use the mogrify command from the amazingly useful ImageMagick toolset. However, as the aspect ratio for the iPhone 5 is slightly different to the iPhone 6 Plus, you need to add a ! to the end of the resize options as follows:
mogrify -resize 1136×640! *.png
This will resize the images exactly to the iPhone 5 landscape size, ignoring any aspect ratio, otherwise you get 1136×639, which just won’t do.
For iPhone 6 landscape size I use:
mogrify -resize 1334×750! *.png
Note that mogrify will overwrite the source file, so I make a directory for each size I want and copy the iPhone 6Plus sized files in.
Just as a FYI, the iPhone 6Plus sized images are 2208×1242 (landscaped)