Not as much as you'd think, about 4/5 degrees. BUT you will kill the engine if you run on petrol like that.
On Blue I have an Iwema ignition processor that adds 4 1/5 deg on LPG and it makes back up about half the power loss.
On the LSE I don't loose any power
On RRR we fitted a processor along with the multi pooint (like the LSE has) and it makes little difference, again LPG doesn't loose power.