Takamura wouldn't be doing it if he felt he had any other choice. They've said over and over again that it's hard for him to find sparring opponents in Japan, and that Japan's weight classes only go up to middleweight. As the coach with decades of knowledge and experience, it's Kamogawa's duty to advise his charges as to what is best for them individually. Instead of Kamogawa making trips abroad with Yagi to look for opponents, it would have been smarter and faster to send Takamura abroad instead.
Safer too. This is just manga, but in real life, drastic, rapid weight loss is never a good idea:
High school wrestler learned training dangers firsthand