In my case it will not effect MOT.
In the Netherlands you can alter/remove the cat if the car is before building year 2010.
However, you must notify the register (RDW it's called over here) that you've made a change regarding catalytic converter.
Now, since my car is a grey import, the nice chaps at the RDW made an error when the original owner imported the car.
Obviously she (yes, a woman imported the car) did not notice the error made; there was noted the car had no cat installed (but it definitely had)!
At that time there was no penalty or other problem to drive the car on Dutch soil.
Currently an A2 diesel with EURO3 classification is environmentally polluting no mather if it's got the cat installed or not.
Then again: if a problem may occur; I've still got the cat and made it so that it can be installed back with a blink of an eye.
Sorry for the long text....