From what I've seen (and I've spent the majority of my time in Japan in Kobe, home to the "Yamaguchi-gumi", the largest Yakuza group in Japan), the Yakuza don't really modify their cars... they just get around in the biggest black sedans from Japan or Germany that they can find... with single digit number plates of course.
As for VIP, Umeda in Osaka is a good place to see some crazy VIP style on Fridays and Saturdays, and I'd agree, the younger guys do get around in older Celsiors and Presidents, and tend to be more extreme in the wheel and audio department. The "late-model VIP" guys are usually connected to a shop (ie: Junction Produces #2 customer) and have huge cash reserves OR (and I say "or" in a very general kind of way) they *modify their Dads car*... I've spoken to a couple of J-guys about this, and it seems that this is one way they get around the huge expense of owning their own late model VIP 22-inch monster.
As for American immitation VIP style? I hope you're referring to Yank tanks, because if the Americans have stolen yet another Japanese style and calling it their own I'd be pretty disappointed...