For anyone still interested in this thread, here's something I've played with recently, and intend to continue playing with several more like it:
It's a UCF10 Celsior running straight gas injection (no petrol), with the stock ECU and wiring still in place, and a little extra help so that the timing and fuel can both be separately altered. Compared to the standard car, on LPG it makes almost 30% more power, with the only other change being a full exhaust system.
It hasn't been running long enough to be able to measure any VSR issues, but it is something I'll be keeping an eye on - not that I believe it's going to be a problem.
I've also got several other UZ (and other) LPG projects on the go at the moment.