I have a Jaguar XK and had mine LPG converted in 2007, the first one in the UK.
When I told the Jaguar community they were up in arms (XKEC & JEC) but I reminded them they were probably the same naysayers about superchargers and diesels in jags
The book combined cycle was 25 but after modifications (intakes/exhaust/head/lubricant) I increased this to 39 then had it LPG converted at that time LPG was 33ppl (now 67ppl) Even so I still can managed 350 miles on a £40 fill of gas and would imagine that only supermini's could better that type of economy?
No problems 70k miles later but my children of 8 may find the rear seats a little snug in future years..hence my prospective purchase of a 400h in the next two months....which I may convert.
I've read a couple of other posts that indicate problems with LPG and the hybrid system but think they are misinformed given that I have spoken to 2 Prius owners who's cars are LPG converted without issue!! Cars with soft stems/valves can always fit the lubrication system to overcome VSR...question you have to ask yourself is:
"Will I be doing 20k miles per annum?" - if no don't consider the conversion as it's impractical.
You should not notice a drop in performance, you will if you get an engine of less than 2l converted but since you rarely use the top-end of a large engine the difference will be no more than having your air-con on full blast.
Best of all use a reputable company with a website that includes testimonials who sound knowledgeable when you speak to them....avoid the "ooooohhhh that's going to be expensive" brigade
Good luck!
Mark