For those spectating, here is a great post I found that tests multiple combinations of HC heads, intake modifications, and plugs. Tests include a Thomas 7:1.
http://www.rarecarrelics.com/tests-t...ord-engine.php
Bottom line seems to be that HC heads produce significantly improved performance by themselves at low RPM, up to about 1500-2000. That makes them a good investment for better performance in stop-and-go traffic. Once you get past that, other limiting factors become dominant, most prominently the intake and exhaust constraints.