If you think of Japanese writing as a whole (kanji, hiragana, katakana where needed) then indeed you can encode more information in less space. Which is easy to see if you compare the Japanese sections with the English sections of dual-language user manuals (those who actually include exactly the same amount of information of course). The Japanese sections are about 30% shorter than their English (or any other language written with Latin letters). One manual I looked at was 60 pages in Japanese, 90 in English, including illustrations (same on both).

But since you need more bits to encode a single character, at least in most common encodings without inventing a custom one, it's not really much more efficient.