Well that shows how much you know, mitsubishi (aka bits-are-missing) are poor candidates for lpg as they have softer valves and are at risk of valve seat recession. Actually i think farmers work with red diesel.
As i said above BMW themselves are working with lpg technology and aim to release a liquid ignited lpg car in the future. Diesel has had its day and as its small particulate emissions become increasingly linked to cancer, so will the tax rise on it - and so it should
There is no noticeable power decrease, although it is documented to be 5% lower power and 5% lower fuel economy but i could never tell any difference in either case.
Its true the 318i is already economical but it becomes even more economical when you pay 65p a litre.
Normally i would change my car every 3 years, but the money i save on petrol has gone into my property instead, and i don't forsee changing my car in the near future