Our experience of LPG conversions is not good, although it does give my technicians practise in rebuilding engines!!
From memory, and I am not a technician, LPG conversions tend to burn the valves or valve seats. Lexus advise against LPG, and also I have seen one or two absolutely horrendous installations. My advise honestly is to buy a more economical car or leave "as is".
Have seen some good LPG conversions on Land Rover products, but I don't believe anyone has developed a system for Lexus (hence the bodges I have seen). Hope this helps.