I don't recommend a horserush strat with the Japanese.... you only get halfway to 3 pop by the time you're done researching HBR and then you've gotta waste time/hammers/gold getting the horsemen out.... they really aren't faster at doing this than any other civ. The Aztecs, Americans, and Chinese are all faster at getting the horsemen out....
I recommend pumping out a few warriors, teching up to bronze, getting the 100 gold, keep teching to either irrigation or code of laws, which ever you wanna go first, while making most of your cities early settler pumps from the fast growth you get from sea tiles, with some grassland growing mixed in....
get to code of laws, you should have at least 5 cities by that time, preferably more... spend a few turns on all gold, pump out settlers, then get to islands and use up as much of the mainland map as you can...
after that, just get Feudalism and Religion fast, and your +1 Samurai Knights attack is usually no problem for taking down archer armies and even pikeman armies...