yes, performance and mpg wise, the cat is a waste of time..
i too would also like to know, if it is removed from say a 94 t0 96 tdi, will it have to be put back on for the mot ????
I had these exact symptoms on a BMW, It was the thermostat stuck open/ or closed?, whichever it was a cheap part and a half hour easy job, . maybe same on disco, i don't know... get a haynes manual....