For arguments sake, is it something you know or are you guessing? On my previous car, Alfa 33 1,5 IE, I removed cat and lambda when new and drove it for over 6 years and nearly 150k km / 95k miles with no ill effects at all. I was a bit sceptical before this was done – but it works all right and it became a bit quicker though the rev range.
A auto engineer told me that the lambda basically does two things; masseur’s temperature and oxygen content in the exhaust gasses and adjust air – fuel mixture accordingly. The cat only works above aprox. 400 degree Celsius fuel mixture is not adjusted for the cat until that temp is reached and after that the mixture is enriched in order to make the cat clean the exhaust gasses (for which it needs an extra amount of unburned petrol). As the engine’s normal running temp is reached before the cat reaches 400 degrees the ECU should be able to optimise the mixture for non-cat running.
Well, the 33 run an earlier version of the Borch ECU so can work in a different way. The engineer told me that a current is send to the lambda which works as a resistance; below around 400 degrees no current is returned to the ECU – above a current between 0,1 and 1,5 volts is returned according to temp and oxygen content.
When refitting the cat and lambda (before an official road worthiness test) the car needed about 200 km for the ECU memory to adjust, but afterwards went all right and was tested to be within exhaust limits.
It could be interesting to hear from someone with experience on this with a 155.