You would have to do it properly at an exhaust place, I'm having mine done tomorrow... I've currently got a sleeved Milltek decat fitted to my Milltek exhaust.
The Milltek exhaust is wider where it joins, then it goes back down to 2.5". This allows the decat to slide partly in so that it is sleeved.
I'm having the cat put back on, the olive join chopped off and straight pipe welded on. My Milltek is widened as mentioned so will allow for this, I don't think yours will be if it has the olive join as standard but it could be adapted at a decent exhaust place. It seems that all 182 systems that aren't sleeved blow :dapprove: