The direct bolt-in rear axle option from The Driveshaft Shop is capable of handling up to 800HP and is now available for the 2004-2007 Subaru WRX STi.
This is similar to their Level 5 axle, except the outer CV and internals are made from Billet Chromoly, while using the original spline size to fit the stock hubs. To make sure the center bar was up to the task of hard launches, autocross and rally demands, they used the new 300M material from Sweden. This material has proven to be the strongest and most resilient bar material they have used to date. The inner CV changed from a tripod design to a full ball and cage design with a larger radius to the spline plug for added strength.
Note: These axle are for the 2004-2007 STi models ONLY