I would suggest that you consult the link to the Tutorial Library in my signature, especially the P-51 and Spitfire guides. In the engine management sections, I do explain what engine limits to respect at all times and what to do if you exceed momentarily these limits. The truth with warbirds is that there is no single engine setting that will suddenly make you outperform everyone else. Each power setting needs to be used in a specific situation (combat, takeoff, climb, cruise). It's useless to go for max power when you are cruising unless you are pursuing a target you've already acquired.
Sure, some people try to milk every additional mph of their aircraft by manually closing their radiator just enough to gain 5-10 mph more... but the lack of radiator position indicator on either aircraft makes it more a safety risk than anything the second you need to manoeuver.