So you mean to say, the FF cone has a fixed opening and the tap one is variable??? As per calculations by fellow PW'er agoodfriend, the FF cone is about 30-50% reduction in Area (not dia) and hence the tap one can do as much justice as a plastic one. May be at high speeds, the cone shape would avoid turbulence at the point but I doubt it will effect performance as the air is yet to pass from filter, venturi to TB.
About the second part, if the carb is serviced, did you check if the car was doing the same thing after servicing. Or are are you noticing the rich mix after some time. I would say, it wasnt set properly in the first place. Ask your mechanic to tighten the screw (lean out the mix) a little more and observe the results.
18 Deg Advance on CNG is close. Try to bring it upto 19-20 BTDC.
Regarding FF venturi, I have read on PW, its not as good as the burner type. People say burner type venturi has its pros. I wont have much experience in that. The FF venturi causes a slight jerk when the secondary opens on CNG. I think the CNG is only metered by engine RPM by the kit alone and not by air flow. You may want to read the "How much this venturi costs" thread to learn out the benefits.