I installed my digital radar two years ago with the mast down. It was a new installation so new mounting hole in the mast for the cable, tapping holes in the masts etc. It is low enough though to use an extension later to install just above the first spreader and above the deck/steaming light. If you had a makeshift scaffold would save you from being a contortionist.
The Cat 5 cable is functional and I have used it for testing and temporary use, but there are several significant differences. First Cat 5 cable is solid wire and not the best choice on our boat, Raymarine is stranded. The Raymarine has a metal housing which may provide continuity of the shield which Cat 5 does not have. The waterproof outer housing at the chartplotter may be an advantage, as I did get moisture in the connector at the back of