LPG has a higher RON rating than conventional fuel, think its about the 110 mark, so when properly tuned to suit, you will see a marked improvement..
on a duel fuel setup you won't, in much the same way as bad fuel will make a car run like crap, fuel thats much better than what the car is tuned for will have the same effect..
so if its tuned mainly for 95 ron, adding in lpg and not tuning it to suit will see a downgrade in performance when on lpg..
as for it in a rex, never seen it done, there was a time when they could not get the pressure up to suit efi, and there was also other issues of it having to much pressure and causing the gas to freeze up..
http://www.gaspoweraust.com.au/ are the people you need to speak to, they ran a drag car on lpg, it'd cut 8's so they know what they are on about..