The Atomos Ninja Star is a $295 HDMI recorder that measures 3″ wide, weighs 3.5 ounces and records 10-bit 4:2:2 ProRes to CFast cards.
It features no display, which makes it an easy companion for the GoPro. However, it also has an HDMI-out port, which allows you to loop the signal through the Ninja Star and out to a monitor for composition or review. As a result, it works well with other cameras where you need a compact rig.
Whatever camera you are coming from, the Ninja Star will bypass the heavily compressed internal codec and provide much cleaner footage.
The Atomos Ninja Star should be available soon here at B&H Photo.