An inch will make a difference in the speedo reading. Since Subaru speedo's are already not known for their accuracy, I doubt it will make a huge difference.
I think the speedo can be recalibrated with a Cobb AP streetuner, but that is some bucks just to make it read right...unless you want more power, then it is more than worth it.
FYI...I currently run a Cobb DP with AP stage 2, with the stock catback exhaust. It is still whisper quiet on the highway, but there is a nice little burble from the exahust at low revs. With this setup, I've pulled on 2 STi's, who I assume were stock.