Skorzeny, I agree, it is a fun topic!
I would add that self-promotion is probably as old as martial arts schools themselves. If you've heard of someone in martial arts, odds are it's because they stood up on a podium and said "Look at me! See what I've done?". Take what you hear with a grain of salt. Even Sun Tzu's famous treatise was a resume/job application.
As for Oyama, his accomplishments speak for themselves.
As far as the question of the Greeks/Romans vs. Japanese, I think you're comparing apples and oranges. The Samurai armies you seem to be describing represent the ultimate refinement of Japanese feudal warfare, at a time when the Greek/Roman armies were already ancient history. I think a better comparison would be the medieval European knight vs. the Samurai.
Or, even more interesting "What if the Mongol fleet hadn't been wiped out by the Kamikazi?"