How much power is lost through a 518

I know details are scarce, because I didn't feel like typing out my life story :D
Pretty much from day 1 I got this car, I've been playing with timing and ways to optimize the engine tune.
I also didn't mentioned the fuel used is LPG (Propane/Butane). That offsets the mileage back usually.

At one point I also went digital with the ignition and installed a MegaSquirt3 ECU, which made tuning a lot easier. This in turn made the engine run better and give back more fun. But.... mileage hardly changed for the better.