Turbos CAN work on CNG. Tariq Bhai at NIPA in Karachi has tuned a few CNG cars on turbo. His own Starlet GT 2E-T used to run on CNG. And it was pretty quick. Left me in my petrol city in the dust!
However, its quite a ***** to turbocharge a carby. CNGing a stock factory turbo car is still feasible ... turboing a carby khyber then cng. Nah bhai.