(Do you know for a fact that firing the spark plug on the exhaust stroke is not costing you power?
How did you reach this conclusion?)
I've had Harleys for years they have always dry fired on the exhaust stroke , even the evo Harleys and Twin cams with electronic ignition, I have seen guys spend hundreds of dollars back in the early 2000s for split fire ignition with what amounts to zero performance or fuel mileage increase, you need four things to make a combustion explosion, fuel, oxygen, compression and an igniter (spark or flame), on your exhaust stroke your cylinder is full of a burning fuel & oxygen mix (flame) , (put a short straight pipe on and you'll see it),your spark event could not cause combustion ,the flame is already burning and the exhaust valve is open so it couldn't make combustion if it was a fresh oxygen and fuel mix ,it would need COMPRESSION, people who spend a bunch of money are over thinking , trying to divide that electrical spark in an attempt to get a hotter spark on the compression stroke and would be much better off just getting a hotter coil or running duel spark plugs, if you have a slow burning fuel you may help it by advancing your timing .