Brewdog

I have a 2012 Grandsport with stock exhaust. The stock exhaust is 2.5" all the way back correct? I'm looking to get a some Kooks 1 7/8" headers and 3 inch X-pipe. My question is, does the part of the x-pipe that connects to the axle back system neck down to 2.5" so I can use the factory mufflers? If so is there a different version that is 3" all the way back? And if I go that route I'm looking at replacing the axle back system in 3" size as well correct? What would the performance differences be between staying with 2.5" axle back versus 3" all the way out?

ptroxx