I've been looking into this a bit lately (as you know
) and my conclusion would be that for the best results, run a high octane petrol (optimax would be the best) occasionally, and also run LPG for the economy it provides, most of the time. Now that you're running all high octane fuels, you should be able to raise engine compression without the risk of knocking (helped of course by the knock sensor as well).
The only problem with this is that it may cost a lot to raise engine compression, and I'm not sure where you could go to get this done. Also the EF engines already run 9.3:1, which seems to me to be far too high to run plain ULP (92 RON, is it? Pretty bloody shit petrol in australia anyway :( ) without the knock senor retarding ignition a lot..
I think what I am trying to get at, is that running the highest octane petrol possible will still be a lower octane than LPG, but should see great results on your car, especially if you've been running ULP so far. Also, running a high octane petrol keeps the engine cleaner and gives better/the same economy because you get that much more bang for your buck (assuming you don't get a heavy foot cause you can feel the power
Also, if you're running something like Optimax, theres a voice in my head saying the engine runs better at 14 degrees BTDC than 10 Degrees.. You might want to check up on that one before you try it :)