I've shot both "Gamekings" and "Matchkings" at 500m Fly and silhouette. If you've got a load that works for you, use it.
Just go and shoot with the load you know and trust. If you reckon you can shoot 15mm groups @ 100m with it, the load should hold up at 500m.
However, if 15mm is your best, one off group, ( be honest with yourself ), be prepared to receive a reality check. It don't take much to throw a shot @ 500m. You will definitely need a good spotting scope, 'cause you are not going to see your shots with that rifle scope.
The fly shoot is a good way to find out just what you and your gear are capable of.
Go and have a crack and remember rule no. 10,
Rule 10 on fly target states ANY COMPETITOR FOUND NOT ENJOYING THEMSELVES WILL BE DISQUALIFIED. 