Sir, in my humble knowledge; i-DSI is a high compression engine, so gas works so fine there. And also: lower octane fuel produces knocking in high compressions. Unfortunately our regular fuel is 90 octane where as CNG is 135.
High compressions also leave unburnt fuel, that is burnt again in i-DSI, so any impurity in fuel will significantly effect it & also is a serious threat to the catalytic converter, as far as I know.
Now, we know gas is afterall: pure, contrary to Petrol which comes even with official impurities: E-10.
Lambda system is good, it offers better acceleration but won't give promised milage. It will cost you a lot more, in addition.
Air control will increase the acceleration (if properly set), but will reduce the overall speed with an increased milage.
Addition of performance plugs is good, especially on CNG.