If you think they need large signage for traffic, then you don't go in the Bakery much.
Seriously, that line is pretty much close to the door from open to close already. Traffic isn't an issue, especially once people start seeing Starbucks cups/wraps coming out of the Bakery. I'm guessing the most intrusive signage is going to be a Starbucks logo on the map. Seriously, count how many Edy's and Coke logos are all over that end of MSUSA. A themed Starbucks sign or two isn't going to ruin anything. And no, a Starbucks cup isn't going to break themeing any more than a bright red cup covered in balloons, either.