well, here's the way to think about it. don't think of the air from the turbo as "flowing quickly" through the intercooler and piping... instead think of the intercooler piping as a pressurized air storage tank. you are pressurizing the intake. that having been said, to reduce the possibility for pressure drop in the system, in our shop we typically try to match piping diameter to what it comes off of and what it goes to. for example if you have about 3 feet of piping needed to connect the intercooler to the turbo and the turbo outlet is 2" and the intercooler inlet is 2.5"(typical setup) then you would want to run 2" piping off the turbo for about 1 foot then blend to 2.25" for a foot, then blend to 2.5" to meet the intercooler. for optimum performance you will want to evenly graduate piping diameter in this way. same goes for the piping to the throttle body.