Run flats are crap! And you'll get an even harsher ride.
Not sure about LPG. I have a mate who runs a Volvo on LPG and it's a lot cheaper to run. It's dual fuel, half LPG, half petrol, when it changes from LPG to petrol the change in performance is not noticeable, but then again, it's a 1.8 Volvo! You'd probably see the differnece in an LCR.