I highly doubt that the material will be better/different in the US kit. I believe there would always be variations in quality during manufacturing. Especially, in the earlier kits, I think you would see more of this as the manufacturer was fine tuning the process.
The bottomline is this: you pay $100 more to buy the US kit, and you will get a set of tires + Mugen tech support. Is it worth $100 more? To me, no. But, to you, maybe.