We'll never solve the question of 'to relieve or not to relieve' - there are more opinions than bung-holes in the flathead world (which makes it fun - you get to pick your fortune teller and make up your own mind).
One thing I do believe is that in higher RPM engines having a relief has to improve flow - or Harley would not have done it for 15 years in their racing KR flatheads. In my opinion there is no flathead engine that had more development and dyno time than Harley and their racing KR's. I've had numerous sets of cylinders and heads and my neighbor built a lot of their race bikes. All were relieved - some 'stepped' - as the heads were stepped and in their larger roller cams, they ran dual profile cams. Nobody has achieved their horsepower per cubic inch - naturally aspirated and on gas. With that said - these were highly developed versions of the old 45 CI Harley flathead - our flatheads have nowhere near the optimized port sizes/designs and flow these guys had.
Here are two pictures for you to ponder - two different styles of reliefs:
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