The P-51 can most certainly turn with the 109. Watch some of t4trouble's P-51 YouTube videos. Or, watch some of Schnarre's (who is an excellent 109 pilot) turn fight with Trouble. Schnarre admits himself that Trouble can get around on him 50-60 percent of the time. There is just a proper technique that it requires. Watching him fly it seems like he uses rudder coordination to great effect.