NAF

I am currently using BPM twin dump downpipe and 3 inch center pipe mated to STi muffler which has some restriction inside the muffler removed and whole system is 3inch and catless.Turbo is Garrett turbo and has been running this mod for nearly a year now.
Now I am thinking of running 2.5inch exhaust system all the way from downpipe to the modded STi muffler.Question is will a 2.5 inch exhaust system cause some loss of top end?Or the loss is minimal?I am doing this due to authority here is cracking down on modified cars and are targeting WRX with big and loud exhaust.I would really like to hear your comments on this.
Thanks in advance.