scooby doo 99

Ok so i thought that i had finally made up my mind which system to go for (nur spec r) but have since found out that it tapers down from 3" to 2.5". I will be getting the car remapped when a new system is on, am i better off going for a system that is 3" all the way through from the turbo or does this reduction in size not make much difference? I have a MY55 STI, which decat down pipe would you recommend? i was thinking about a japspeed but not sure if all down pipes and systems are compatible.