Often companies who manufacture in China send in their best engineers and top production staff along with tooling and machinery. Huge automive parts companies from the US have done just that, Like TRW for instance. So made in China may or may not indicate a negative experience with a product. One thing is for sure, it's hard to avoid a 100% not made in China product. Not that I'm a fan of this, it's just fact that at least some component of many items not made in China has that component made there.. I have no idea about Yamahas policies on this though.
That said the MOX8 has weighted keys and has their latest sound engine. I don't know a lot about Synths but if I were looking I'd almost surely go with the MOX over the MO with the MO technology being introduced in 2005, figure development time, that's getting old now.. The Synth catagory is one where I might look at a pro level Korg as well ( actually I have looked and was pretty impressed). I know you said you were only interested in comparing the Yamahas though.
Not much help I know but at least you have one reply !!
David