I see you've managed to look at hockeydb for this year only. Kapanen has never been close to as productive as Johnsson was last year on a per minute basis (which was very impressive fwiw, Johnsson had low end 1st line production in less than 14 mins/night), and Johnsson's career has been notably more productive as a whole. Johnsson was also slowed by an injury earlier this year, appears to be getting back to form now, has 5 in his last 7 games and is looking like himself again.
I'd encourage you to watch the two guys play, it will get obvious pretty quickly. I'm not saying that Kapanen is a bad player either, he can be used in all situations and will be a net positive in any of them, he's just not offensively efficient & I don't see it improving because it looks like a processing speed issue to me. If he's ever going to be an offensive star, it will be the way that Jason Blake did it - high volume, low conversion.