My understanding (for what it's worth) is that -- although rapid changes in diameter should be avoided in order to minimize turbulence -- the bigger the pipe the better when it comes to a turbo (since it's force fed). So if you've got 3" all the way through, it makes sense to use 3" downpipes too. The exhaust gases at the turbo are already so hot, the flow is less restricted by density than it is by pipe width.