Well, i must agree, that i don't imagine him sounding "respectable" as you put it...
You're right, that his attitude in Vol 16 seems to change a little, as if he would indeed let go of all this "madness" (if that's how you can put it..?)... But, well, he seemed concerned for his fellow workers... i mean, he didn't seem to want to leave the others with the job, guess he felt a little too attached to them, and don't feel alright to just leave them for his own good
That said, i couldn't imagine him with this serious tone, with some kind of wacky voice, hehe
Iwata though, i think they did an outstanding job with him in the Anime version of the series...
I feel the manga is in every way superior content wise and story wise compared to the anime, but Iwata's voice seemed to fit just fine into the manga as well... one of the few things i think is equal to the manga
His voice simply didn't sound TOO goofy, and still had that similar attitude as we know him from the manga (well, somtimes it got a little exaggerated to my tastes, when comparing to the manga, but his voice still fit) Note that i liked the original japanese, as well as the english version, this whole opinion might as well count in both these voices...
Uhm, but, it's about Sumiyoshi... so, maybe i shouldn't make any big deal of Iwata's voice