If it is really good then yes it should. The Flat White will definitely bring in people to at least try it. The Flat White is really popular in other countries, specifically Europe, and is supposedly a more refined version of a latte. I know that I am interested in trying it!
The introduction of the flat white will help Starbucks' sales. Customers will undoubtedly be drawn in to try the new coffee, hence sales should initially increase. Consumers attracted by the flat white are also more likely to spend money on the existing snacks and beverages already for sale in Starbucks, thus further increasing revenue.
Yes, the introduction of the flat white will help American Starbucks sales, because Starbucks has a history of being on the cutting edge. I really miss their drinking chocolate, and I would go there more if they brought that back too. But Starbucks' new products are always tasteful and they will bring in more customers.
Introduction of the flat white to America will not help Starbucks' sales because there are so many others in competition. In America, people choose to buy what they like and no discounts or advertisements will change their mind if they have no interest n the product. Time will often tell.