Oldies but Goodies,
The most obvious opportunity to enhance cartridge performance is in handloading cartridges with origins dating back to the late 1800's to early 1900's. Some began life as low pressure black powder cartridges, others as low pressure smokeless cartridges, almost all were produced when firearm and cartridge case designs were less robust and material strength was inferior to the materials of today. Two of the most well known cartridges that fall into this category are the .45-70 Government and the .45 Colt.
In conjunction with specific strong firearms, and supported with detailed guidance from mainstream handloading manuals, these cartridges can perform impressively. A .45-70 Gov't 350 grain round, safely loaded for an original Trapdoor Springfield, tops out at approximately 1,700 fps. Loaded for use in rifles such as the Ruger No.1, the same bullet can be pushed to 2,100+ fps. The .45 Colt, loaded for the Colt Single Action Army design is limited to approximately 875 fps with a 250 grain jacketed bullet. The same bullet, loaded for the Ruger Blackhawk and similar can safely attain 1,200+ fps of muzzle velocity. While I do maintain my own handload data, mostly a crumpled up piece of paper tucked away in back pocket, these performance references were taken from the Speer #14 handloading manual. Very similar data can be found in manuals from Sierra, Hornady, Nosler, etc.