With engines of low V/E (that is poor air flow) you need to run
more boost than a normal overhead valve or cam engine to get the same level of power.
So you need to have less ignition advance to control the heat in the combustion chamber, and with less advance you start to burn the fuel in the exhaust and create a vicious circle of heat control in the entire system.
At least with a flathead combustion chamber, to get maximum power you don't need the advance of most engines.
I would think that coating as much as you can afford is very helpful and worth the expense.
