Can't see how you'd get any performance gain from changing a lambda sensor unless the existing one is faulty. A simple emissions test will quickly identify if the sensor is working properly (it's part of the annual test in the UK); if the readings are within limits, then there's nothing to be gained in performance or economy by changing it. If it's out of limits, it'll likely trigger the EML light anyhow.
Also you won't get any gain in performance from modifying the intake except when you're operating at full throttle.
What may help (not so much with performance but with improving cold starting and running) is to fit iridium spark plugs - the original Fiat fit parts are rubbish. Probably the easiest, cheapest and most worthwhile mechanical mod you can make.