I disagree,

As history has proven, roman armies work best in simple head-to-head battles, but i do not think that you have taken the full potential of the japanese in concideration

U might think that the battle will be a headlong japanese charge into the romans (banzai no-dachi madness), but in reality, the japanese used many different tactics to overcome their enemies. One thing is, that their vast veriety of agents would be able to asassinate roman generals, and obtain informatoin on the enemy better then the romans, and another thing is that the japanese used the terrain, weather ect. to their advantages in mulitple battles. If the japanese attacked a roman army on the march or at night, the romans would definately get beaten. If they surrounded the romans and forced them into breaking their deadly groups so the soldiers would have to fight as individuals, the japanese would beat them (refference to hannibals campaigns)

Now... i agree, that the romans would win as long as they would be able to maintain their formations, but when faced with mobility and superior tactics (as used by for example oda nobunaga, uesugi kenshin, and tokugawa ieyasu), the japanese would easily gain the upper hand

So in my conclusion, the roman army has to be forced into not being able to fight a frontal melee assault..... and the japanese were good at using "cruel" tactics