Please, do not remove the "silencer" from the Nur Spec R centre section. It's not their to reduce noise, but to tune it to sound better. A guy in our club cut that bit out and it just didn't sound the same
The 2.5" section of the Nur Spec centre section can't be more than a few inches long. Any decent exhaust place should be able to cut it off and replace with 3". I had Hayward and Scott do it at the same time they fitted my 3" downpipe back when I had the black WRX.
From what I remember, centre sections don't fit the Nur Spec backbox because it's flange is at a different angle? Something like that anyway..
Hope that helps?