The article says 1W/core is a limitation of maximum optical power density in the fiber itself so if you want to push 240W through, you will need around 240 cores no matter how long or short the cable run is.
Efficient? Going from electrical to laser is at most 30% efficient and then back to electrical is also about 30% efficient at best, which means about 90% of input power turns into heat at either end. Not something you'd want to do unless absolutely necessary. To get your 240W out, you'd need to put about 2.4kW in.
If you want to transmit electrical power and there is no need to be completely electrically isolated in every possible way, use appropriate gauge conductors and Y-delta transformer setups to avoid forming ground loops.