okay lets start with the prop size a 13-5 prop is about equal to the 12-6.th the 13-5 will do better verticals but be slower on a level fly by than the 12-6..overall the 12-6 will be faster a better choice than a 13-5 if the plane is on the heavy side.sounds like the engine has not broken in yet or is being run too lean.there are plenty of world models p-51 being flown with the stock size spinner so do not worry about that and yes most of the thrust is from the tips as the roots of the prop due to the diameter are turning very fast so being covered by a spinner is not an issue.borrow a tach and see what rpms the engine is turning with the two props you have.what percentage of nitro are you using..is the engine mounted upright or inverted.you may be running it rich like a said because of a cooling problem .so a picture of your installation could help.