How much ID change?

Smaller ID of the pipe means thicker walls.. so the pipe will be stronger. It also means more backpreassure in the exhaust system... that can rob a bit of power. You can reduce this a LITTLE by beveling the iside of the pipe. (VERY CAREFULLY... and clean out the metal filings completely.) But would it be worth the effort? (probably not)

You probably won't notice the difference the smaller ID makes unless its fairly significant.