Indeed the Boston pianos, although designed and marketed by Steinway and Sons, are made in the Kawai factory in Japan. There is a slight price difference in the smaller models with Boston being more expensive, but at the top end of the range of models, the Kawai pianos are more expensive. At least that is the situation here in Auckland.
As the Boston is a new line of pianos which started in 1992, we can only speculate on the durability of the instrument. However, there is a the "Steinway Promise" which pledges a full refund of the purchase price of any Boston piano within ten years, should the owner decide to upgrade to a new Steinway grand. We may infer that Steinway and Sons has put much faith in the enduring quality of the Boston.
Kind regards,
Robert.