Hey, I'm trying. Clearance is important for timing and overlap at intake/exhaust TDC, when both valves are slightly open.
Most of the power increases over the production of these engines were due to changes in the cam profile. I see lots of people talking more fuel, more boost, and no cam alterations so it's an overlooked part. I know going to a Delta torque grind in my Subaru EA81 actually really woke it up, I am also running a Delta custom grind in the 300 in my van with great results.