Hi Robert,
I have similar set-up, but on 2.7 engine (with manual).
As I installed the long tail headers and downpipes with 200 CPI cats - the car run pretty nice compared to old OEM exhaust.
On getting ready for MOT, I swapped new Bosh spark plugs (with 4 leads each but non platinum). Car run a tiny bit rough as of what I could feel.
Went to the MOT and failed due to too high CO%.
Also the CAT efficiency CEL showed up couple of times a week. So I swapped also all Lambdas. Car still did run a bit rough.
Next I installed new and NGK Platinum spark plugs (NGK BKR6EQUP). Now the car run very nice.
Cleared the CEL's with Durametric and went to MOT.
The inspector looked at the exhaust after I told him what I had done to improve the CO%.
Initial test was again too high CO%, but then he heated the CAT's real well for 5 minutes on approximately 2500rpm and then the car had CO% on 0,08!!!
Reasoning he told was that the 200 CPI 'race cats' are metal based and required more heat to operate than ceramic CATs (he said that he has same style 200 CPI metal CATs on his Impreza...).
For next MOT he told to drive the car for 10 minutes with very high rpm (actually told to drive against the rev limiter
) immediately before the MOT to get the CATs very hot. Then the CO% should clear easily.
Anyhow, I'm planning to take my car to same inspector also on the following years to come
