I bought a VT on factory gas about 5 months back. Its a standard V6 not an V8. Performance is still quite good although this is my first commodore so dont have much to compare to (previous was an '89 TX5 2.2 turbo). Factory gas systems are dual fuel so you can always flick over to petrol if you want a few extra HP.
Any misfire problems due to bad leads or plugs can result in backfires back through the airbox, sometimes powerful enough to destroy the airbox and a few sensors. Newer airboxes are ribbed for strength and tend to pop open easier when a backfire occurs but can still rip apart with a big backfire. LPG backfires sound nothing like your regular "through the exhaust" backfire, they simply sound like a dull pop rather than a bang, if you have ever hit a small animal like a rabbit it sounds something like that (sad face). When the car is badly tuned they will most likely occur during starting or under load. I actually tend to give the starter an extra second or so of crank after the car starts to ensure that it doesnt stall and backfire. Some people prefer to start on Petrol and then flick to LPG, I cant be bothered doing that. Its also best to keep the car well tuned.
My car actually runs worse on petrol at the moment (hesitates on takeoff), but this is likely due to dirty injectors, I cant be bothered getting them cleaned though as I run on GAS almost all the time. Its recommended to run petrol occasionally to lube the system so I hear.
The factory system is also setup so that it sucks a bit of petrol even on gas when the engine is under heavy load, this gives you a bit of extra power when needed. So its important to keep at least some petrol in the tank to prevent fuel pump damage (or so I hear).
I think my LPG tank is about 75 litres and I get around 550km of mixed driving on a full tank, gas prices here in Coffs Harbour NSW can vary by about 10c between suppliers... Ive found Woolworths+ Service station to be best value at about 44c a litre after 2c docket discount. LPG seems to stay at a fixed price rather than fluctuate based on the day of the week.
Apparantly gas efficiency is not quite as good as Petrol.... but with the price of LPG well under half that of Petrol I know what I would prefer in the current market. It is also much better for the environment as you are likely aware.
Converting your car is likely to cost you several thousand, you need to calculate how much you would save each year based on current usage, and then consider how long you will keep the vehicle. As a rough figure, assume that you will at least halve your current fuel bill. If you are intending to sell the vehicle within say 2 years and wont actually break even within that time then the answer is probably not to bother converting. However adding gas to your car is likely to increase its value somewhat, probably not by as much as it cost you though.
In my opinion, LPG vehicles are likely to sell somewhat easier to a certain market considering the current petrol prices. The guy I bought my car off had many inquiries via his Tradingpost advert and Im sure he could have sold it many times over (about 14 calls the day the ad was released, I bought it via the TP website prior to it being released in the paper), people were willing to fly up from Sydney to Coffs to check it out, not bad considering how many VT Commodores (petrol powered) are for sale at any one time.
If you do long outback trips you may benefit from the added km range due to having two tanks of fuel onboard.
The only major downside as I see it is bootspace reduction, the LPG tank takes up a lot of space and if you have a family this may be an issue, there is still room for quite a bit, pram or two small suitcases etc, but you may certainly miss the space. You will also lose the ability to put long items in through the boot and into the cabin via the folding middle rear seat. So say byebye to sticking skiis in the boot lengthways.
Hope this helps.
Cheers,
Graham
