No, it isn't a formality at all, the rental companies are not prepared to deal with anything other than the International DL. That being said, once when I was unaware of the requirement, I was able to talk the rental agent into renting me a car with my California DL -- but it wasn't either easy or fun (especially with me not knowing Japanese). That was more dumb luck than anything else, so I'd strongly disrecommend taking that approach.

Incidentally, regarding going flat out, there is no freeway in Japan posted higher than 90kph, I believe, although apparently there are no serious fines until you exceed this by at least 30kph.