I would say 1) no signficant performance increase unless the 390 is putting out around 450+ HP and 2) yes, 3" pipe costs more than 2 1/2" pipe. How much depends on the shop and how they price their materials.
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.
