You do loose a little bit of power.
If u have a read of a book called. 'The essential Holden V8 engine manual' by Larry O'toole the u'll be able to see the difference gas makes.
a straight up swap without changing the petrol timing resulted in a loss of around 10flKwts. Once u tune the gas throttle body and timing then u get back 7flkwts, 166.
They where able to get 194kwts out of a set up that includes: an increase in comp, 2 1/2in mandrel exhaust, K&n, CAI, and a cam (duration @ .500 IN 200 EX 215, cam lift .046', lobe lift IN .452' EX .472"). The set-up also had std headers.
its was done by Gas Research Australia on a VN 304 with 140,000kms on it.
If u have a good ignition system and exhaust system plus service ur car regularly and use gas type oils then u shouldn't have a problem.
if i was doing it, i'd just go gas only. that way u can use a different inlet manifold, E.g a single plan manifold, and have the engine just run on gas. Also u can take out ur petrol tank and replace it with gas setup.
think about a larger cam with a higher comp and put out more power but still have better fuel economy then the std setup! also once u tune the system to gas or use a gas/higher comp ratio, then usually petrol won't run well, if at all!
anyway good luck.