| What you have said may be true, but I have my mini converted to LPG. I do 40,000 miles a year. Plan to have the car 2 and a half years. The conversion will have paid itself back after month 9.
Mine is an R56 and on LPG on the motorway I get between 37 - 40 mpg.
I have lost my boot space which is the downside. The system can be removed. It's not damaged the dashboard and I had the filler put in the bumper which can be replaced. However, I have known people to fit the refill under the back number plate so it's hidden.
I also have no problem with Mini. It was serviced two weeks ago without issue and the warranty stands (obvioulsy items connected to the LPG wont be covered).
cheers | 
