Lehigh/Knight NFPJ Breach plug using whichever primer fits best in your gun. I really feel this improves not just a clean breach but keeps pressures more consistent. I prefer BH209 but Swiss has excellent shot to shot extreme spread and standard deviation. There must be some reason it is preferred at Friendship competitions.

I would choose .458 bullets with a high BC or maybe the Barnes/Knight .451 285gr Spitfire. Its has one of the highest real world BCs. The The Barnes O 300gr SS has about the best BC in the 458s. Usually BC is over rated but past 200 yards it becomes more important IMO. My best shooting 458 is the 300gr SOCOM with a BC of .236. Knurled in a MMP Orange or plain in a Harvester Yellow for BTs usually yields very nice groups with heavy loads.

Keep everything as consistent as possible including indexing sabots and loading pressure. I found the Knight/Lehigh range rod works very well for serious range work. It really gives you a good feel too while loading. I can usually tell as soon as i use a ramrod if something is wrong or feels different. Extend your cool down time to a solid 15 minutes between shots.

That should be a good start before moving on to more expensive improvement to the gun. A good bedding job would be on my list also