I don't think the hp/cfm level are impressive in themselves. Just how the 393 could squeeze another 81 hp out of a ridiculous small head over the 306, even though I imagine the 248 solid roller vs 212 hydraulic roller did play a big part but doubt it was responsible for the total gain. Just thought it was interesting.
But the 2.41 hp/cfm 393 engine did gain a 168 hp when he did put on proper heads on it, 387 vs 550 hp.