What I am more curious is how much the culture is effecting corruption in China. Because personal relationship plays much larger rule in the functioning of society, which unlike the Western society everything is much more dominated by the legal process and lawyers. I don't think one is inherently better than the other system, so I don't think it is feasible for one system to change to be completely into the other system. But in the long term how will China's relationship focused society adopt it is method in dealing with corruption is something interesting to see.
Actually let's put it this way. We measure pain by its intensity, but biologically the neurotransmitters do not reflect a stronger pain by greater amplitude; instead it's by more frequent pulsing of the neurotransmitter of the pain receptors. (With that said, if you eat something you feel very spicy, this means your NT is pulsing a lot quicker.)
Why did I bring this up? It's the same thing for corruption. How serious corruption is isn't simply measured by intensity; but by prevalence and frequency. A reason why China's corruption is so serious is because it's prevalent throughout the entire society; from top to bottom. We might be more inclined to believe Wen was a nice guy, but of course even if he really was and had no hands in corruption, his family and relatives do their shares of name-wrecking for him. Low-level corruptions occur with officials and businesses, and even between businesses and services you may often have to choke up extra fees. I personally experienced this in Chengdu, where the security guard requested me to pay "tips" for services rendered just for finding my phone which I accidentally dropped. It didn't quite catch me immediately that's how they made side earnings, as at the time all I felt was that he's literally "taking my phone hostage". Businesses often needed special licenses to have a smooth procedure to opening, and there are many various. So with all that said, that's my perception of how serious corruption in China.
As for the culture, it definitely plays a part. Japan's corruption (which my Japanese professor called "pork-barrel politics") came from donations from prefectures and small communities, and then in Japan when politicians accepted gifts and donations, they must do their part because it came from a more traditional Confucian ideology of "returning the deeds when someone offered you a favor." Much of the scandals in Japan's political world often related to this, which is why we also often see PMs in the recent years leaving so quick(not a good thing.) However why Japan emerges as much cleaner than China would have to do probably more with frequency and prevalence; it's not as widespread to grassroots level as it is in China. And of course there's an element of bias too.
Overall, the collectivist cultures and confucian cultures may have some influence due to the bigger concerns with relationship-building. Despite that, it does not mean it's necessary so, as HK and Singapore are proud holders of least-corruption rankings. Ask HK's ICAC and your answer is there. It therefore demonstrates that perhaps proper ethics, training, management, education, income, will all play a huge role in turning things around.
What else which made China ranked there and not lower would also be transparency. Perhaps it really is safe to say that China still have a lot of issues with transparency, which was one of the major factors. This again isn't to say other states are fine as they are; we just shouldn't compare ourselves with the lower.