Will my WRX clutch and flywheel work in a 6speed?
-Yes. The diameter of the WRX flywheel is smaller (230mm compared to 240mm) than the STi flywheel. The bell housing on the 5speed is also smaller. There is less surface area with the 5speed compared to the 6speed but is not a huge difference. There are no issues with running a 5MT clutch and flywheel in a 6MT.

Will my WRX starter motor work in the 6MT?
-Yes. WRX and STi starter motors are the same part number.

What if I don’t have/use a DCCD (Driver Controlled Center Differential) controller?
-The DCCD defaults to open mode which is a 35% front/65% rear power routing in 04-05 and 41/59 in 06-07. There isn’t anything “wrong” with this, but if you are running the R160 with the 3.90 Ring and Pinion the compromise is in an even shorter life of the differential since more power is being routed to the rear. This of course all depends on the driver as some have a knack for blowing parts out even at stock power levels. The options for a controller are: Spiider’s (DCCDPro.com), Neetronics.com, RocketRally.com…

Is there a strength difference between the WRX and STi drive shafts and axles?
Subaru’s use 2-piece drive shafts. The WRX drive shafts are 2.5” longer than the STi and they use a ball-joint whereas an STi driveshaft uses a universal joint which is stronger. The STi axles are stronger because of a larger diameter bar and beefier CV joints.