I have to agree with the above posts that riding through China is a frustrating experience. We (two motorbikes) did a 6-day 'tour' from Kyrgyzstan to Pakistan via Kashgar earlier this year, 3 days of which were spent getting the bikes in and out of the country (you don't know bureaucracy until you've been to China with a vehicle...). The rest of the time we had to follow the guide's jeep.
It is also very expensive, especially for bikers since the guide will need his own car. Rules change every year and our guide told us in the future permits will have to be applied for per province you want to visit, which will make it even more expensive and time consuming.
If you do decide to ride China, join a group like you're planning to do to get the cost down (you'll need to very flexible in terms of dates tho). If you'll organise it yourself, shop around because there's huge differences in price and professionalism between agencies. But personally, I'll leave my bike at home or in another country when I visit China again
Good luck!
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