I've seen them set to kick in when the turbo kicks in, that would make it very efficient for cruising. I've heard lots of great results from running D-Gas systems.
WA is the place to be for getting it done though, we get $3000 rebate instead of the $2000-$2500 the rest get.
As for petrols, don't know what you're waiting for, my 4.0l XJ costs me about $36 do do 400km on LPG (14l/100km, LPG @ 65c), considering it only cost me $400 for the conversion that means I make my money back in 8 tanks
($50 saving per tank). It really is a massive saving!