LPG gas injection in turbo diesel

My husband was recently speaking to a caravanner whos friend has recently fitted an LPG induction system to his turbo diesel Nissan Patrol. Apparently he is getting upwards of 80-90 newton metres of torque extra and uses 2 litres of diesel less for each 100 klms travelled. We don't know how technical he is but he talks about the fact that diesel combustion only operates at 75% efficiency and when a 20% LPG is injected into the combustion chamber a burn of 100% is achieved. The stated cost of this conversion is $3800-$3900. However with a Govt subsidy of $3000 makes this option very attractive and "apparently" this chap recouped his $800 in just over 6 months. Our concern is that we have a near new 4.2 lt auto turbo diesel landcruiser (under warranty). Unfortunately we know nothing of the technical side and would greatly appreciate hearing from anybody who has either fitted this conversion as to whether it is a sensible option having regard to the ever increasing fuel costs.