Autogas 2000 are the experts so if they say no, it really is a bad idea.
I ran a V6 alfa for 6 yeras and it was sweet but to avoid you thinking that it is just a money saver, that MPG figure is wrong. You will get about 2/3 of the mpg per litre on LPG as on Petrol. The calorific value of lpg is lower by volume so less energy in the cylinder when bang happens.
I found it made my car as economical as a diesel when you took fuel costs and mpg into account.
On the plus side, a petrol car is cheaper to buy than a diesel (generally), a petrol car is more fun to drive than a standard diesel (generally), and the cleanliness of the lpg is an environmental bonus. Also it is kind to the engine and the opil so engines tend to last for a long time (the alfa isn't mine any more but is still running fine and is nearly at 250K miles now).
On the minus side, a chipped diesel is a better deal nowadays. The LPG tank takes up too much space, the tank range is generally poor so you are forever filling up, oh yes, and apparently it will melt your 350z valves.
See - you aren't missing a lot