Yes 3" Really is better than 2.5. The best exhaust for turbo is no exhaust, the car already has enough back pressure from turning the turbine inside the turbo. 3inch will be louder but provide more hp. 3inch downpipe with NO exhaust will be loud and provide the best hp.

BTW there is markup at every dealership. Dealers make the most money off used cars because there is no set value and salesman can jack up prices as they please, and also quote yur trade in as having less value than it really has. Your best bet is buying from a private seller, same car just hasnt had 100 point inspection from the dealer . . . . .