Check the after cat lamda at cruise speed the reading should be steady at about 0.7 volts or above this shows that the cat is efficient and burning all the oxygen.

I have seen after market cats that will pass mot test ok but the engine manangment flags a catalyst inefficient code due to seeing to much activity on after cat lamda .

Gas readings on a good cat 0.0 co

lamda 1.020

Some of the after market cat only get the co down to 0.2