I am getting 150-160 KM/h in city Isb/Rwp. and 200+ on motorway. no issue on CNG, in fact very minor and hardly noticeable difference in petrol and CNG.
The only one Issue with Santro and Baleno like other EFI cars as well is that, it has some misfiring issue, but not with all santros and Balenos, so when you go for buying one, check its misfiring issue. Mine don't misfire.
Mine is after Market CNG cylinder with 50 litres while FF CNG cylinder has 14 litres each with three cylinders, 14x3=42 litres in total.