I'm not familiar with firearm legislation or joule/fps requirements and restrictions in Japan. But airsofting in Japan is quite big,heck it is were airsofting orginally created. Many reputable Japanese companies manufacture airsoft weapons such as Tokyo Marui.

But from what iv gathered, Japan was a joule limit of 0.98 joules. Anything over this amount will require a licence and classed as a firearm.

So from the sounds of your post and the 'Walter P99 RAM' that you mentioned, yes you would need documents to obtain such a thing and I would say obtaining such documents in Japan is hard in general and probably even more so for a foreigner.