I agree that Akira Kamiya's performance is the best Kenshiro we've ever had, but the movie came out in 1986, and Hokuto no Ken 2 (episode 110+) didn't finish until 1987. Kamiya reprised the role in the non-canon Dreamcast game in the mid 1990s, and the Playstation game a few years later. He even dubbed Gary Daniels in the goofy live action Fist of the North Star direct-to-video movie!
Kenshiro didn't get a new seiyuu until Shin Hokuto no Ken in 2003, giving Kamiya nearly 20 years off and on with the role exclusively. The whole project reeks of stunt casting, with professional wrestlers and rock stars filling the ranks. It's the only time I'd consider the English dub in the Hokuto no Ken franchise ever so slightly less awful to sit through... nothing against Takehito Koyasu, but talk about poor casting.