Personally my concern on warbirds was not the ball bearings but the gears , the larger the warbird or flying servace to be moved the more I would lean towards metal gear servos .
Example - 60 size warbird , H9 , I use regulat servos , its light and flying servaces are not to big
Then a Pica 1/6 P40 , 100% medal gears with ballbearings , the larger or heaveier the surfaces/plane the more chance you have of stripping a plastic gear under load .
IN the case of the P47 , I think its a relatively light plane but you may wnat to have medal gear servos on the elevator because its a bog plane