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Your S550 Mustang's stock half-shafts are excellent for stock applications as Ford intended. However if you're planning on putting extra power through your pony, upgrading the half-shafts is probably in your best interests.
Building up your Mustang requires making sure all the parts can handle the power. Upgrade your half-shafts before you break your current ones.
Since the Mustang was first released in 1964 they have used a solid rear axle, with the exception of the 1999-2004 Cobras. The solid rear axle is a strong and easy to upgrade unit that works very well for drag racing. The axle shafts of a solid rear axle don’t have to articulate at all, since the axle stays straight and moves as a unit. When Ford transitioned to an independent rear suspension on the 2015 Mustang, with it came the need for half-shafts. The half-shafts contain a CV joint that allows the axles to move independently of each other, as well as up and down with the suspension. Half-shafts are typically weaker than their solid rear counterparts, and will need to be beefed up for the serious drag racer.
Before you get worried that your stock half-shafts will break on the street, remember that Ford tests and designs their OEM parts to last a long time and perform well with stock power levels. The stock half-shafts will be able to handle bolt-on upgrades on Mustangs that run street tires. The biggest factor to breaking half-shafts is torque, especially instant torque, and having sticky tires. Hard launches on sticky tires with lots of power will shock and snap the half-shafts. The constant velocity joints will also wear over time, eventually leading to failure.
When Ford designs a part, they have to consider the cost to manufacture parts and the speed at which the parts can be produced. Stock half-shafts feature cast parts because they are cheap and easy to produce. Cast parts are inherently weaker than billet or forged parts, which also cost quite a bit more money. The current aftermarket half-shafts feature billet CV cups and stars, compared to cast stock cups and inner stars. The splines on them are also compressed when they’re formed instead of being cut or cast, which reduces stresses within the metal, and makes them stronger. Stock half-shafts are also hollow. Hollow shafts are cheaper, lighter, and can be packed with materials to reduce noise, vibration, and harshness. The actual shaft on the aftermarket half-shafts are one single piece, and are not hollow, making them much stronger than the OEM axles.
OEM axles will work well for people running street tires with your typical bolt on mods. If you are going to be running your pony at the track with sticky drag tires, you should absolutely upgrade your half-shafts. Even with basic bolt on mods, the extra grip from drag tires can be enough to snap a factory half-shaft and end your day at the track. When adding sticky tires to the mix, something has to give and it’s usually the axle that breaks before any other component. If you’re going to be installing a supercharger on your Mustang, it would be a good idea to upgrade the half-shafts too. People running positive displacement superchargers should be especially concerned with replacing their half-shafts because of the instant torque produced by that style of supercharger. The bottom line is that if you plan on racing with sticky tires or slapping a power adder on your new pony, upgraded half-shafts are a must!