The 75 is a lighter car than the 155 so yo will get beter fuel consumption, especially around town, although 40mpg in town sounds slightly optimistic
I think Wrinx is right, the whole point of the Lambda is to tell the ECU the CO level so that it can adjust the fuel / air mixture etc to meet emmission levels. If it pulls better, then its probably because your 155 air flow meter isn't as good. My V6 Lambda recently gave up the ghost and its CO level was 4.5%, with a new Lambda it was 0.03% (max allowable is 0.3%). An Alfa 75 with no Cat will have to meet only 1% +- .05 which the adjuster helps with, but I doubt it would adjust to meet the 0.3 that your car must achieve with its Lambda and Cat.
Are the headlights any good on the 75, and what colour is the strip on the boot lid?