I flew an RV-6A with an O-360 for 3 years. It exhibited similar CHT problems as described and I implemented many ?fixes? - RTV sealing of the baffles, filing of casting flanges between fins on cyclinders, altered front baffle deflectors (to better balance front to back air distribution), closing the open inlet ramps, better cowl seal material. All had positive affects, but I still had to watch my climb speeds (> 120 knots) and used step climbs in more extreme conditions (high DA, high outside temps, full load, etc). Due to a cyclinder AD, I did a top end overhaul (new Millenium cylinders). During this overhaul I found two cylinders where one of the compression rings was broken. All cylinders were making compression (one with a broken ring was consistently over > 70 psi, the other in 60s). After the top overhaul, all CHTs dropped by 20-40 degrees.
I offer this history not to scare people with CHT issues - it does NOT mean you have to do a top overhaul. However, everyone should be aware that high CHTs are not always solved by easy fixes, and can be symptomatic of bigger issues.