Mine is dual fuel, it still starts on petrol when cold then flicks its self over to LPG once the engine temp reaches 33oC (I think, may have wrong temp there, but something along those lines) In summer it just starts on petrol, nothing for me to do, it just does it.
They reccomend that you run on petrol every now and then, if for no other reason than to keep fresh juice in the tank, burn up the old stuff, so I use about 20L of petrol a month, and over 100L of gas every week.
Keeping in mind that it runs colder plugs and is tuned to perform at its optimum on LPG, my car only uses 600ml per 100km of petrol more than the fuel economy figures that come with a factory petrol car, but also gets the same economy (13L/100Km) on LPG.
As for power, I really should put mine on the dyno, pitty I didn't before I had LPG fitted to see exactly what gain or loss there was, but when switching between the two, it feels to me to have increased power on LPG, but again, it is tuned for it, but having said that, I may or may not have (lol) had a run UP Anthonys cutting with a MY11 model on petrol, who no longer laughs when he sees an SSV on LPG.