Quote - It is sumou or sumō, not sumo.  I know that this is being pedantic and the other spelling is accepted 'English'.  But if someone said that they saw an ardvark - what would you do? ;)

There's a difference.  "Aardvark" is a Dutch word, a language that already uses Roman orthography so there was no need to transliterate the word.  "Sumo" comes from a language that didn't use that alphabet.  The use of the macron for long vowels is based on the Hepburn romanization of Japanese, and while that's the most common one used it isn't the only one.  In Japan, while Hepburn is the de facto standard, it's often simplified by omitting the macrons and circumflexes.  Odds are if you see a sign in Japan saying "Tokyo" it will read just that, not "Tōkyō" which Hepburn, strictly speaking, demands.  So you're more likely, in Japan, to see a sign indicating "sumo" than "sumō".

This isn't just Japanese, but happens commonly when translating words from one language to another that uses a different alphabet.  Arabic provides good example: the leader of Libya's name is variously translated as Khadaffi or Qadaffi, just to use two variations I've seen.  None of those are wrong and which you use depends, for instance, on what newspaper you write for.