Spec C rear diff in 2005 WRX
 

This is what I have found out so far:

Spec C Rear diff Ratio - 3.9 (R180)
WRX Rear diff 3.54 (R160)

R180 Prop Flange holes are 10mm larger diameter PCD than the R160

I will need a R180 3.54 Ratio Crown wheel and pinion to swap into the Spec C diff to correct the ratio for the 5 speed MT.

The ABS ring on the R180 is 5.2" - R160 is 4.8". I've read the bumps and cut outs on the R180 ABS ring as slightly wider so even with the larger diameter the ABS System shouldn't see anything different between the two over a rotation.
Can anyone confirm?

I will have R180 drive shafts and hubs also. (I have 2 pot brembo rear brakes to fit)

Am I missing anything?

im missing the point?

I race the car on track, thought process is that it will give better rear end grip (more so in the wet to the dry), reduce some under steer hopefully. Also allows me to bolt rear brembos straight on no messing about with the back plate or handbrake shoes. And when I decide to swap in a 6 speed I will only need the front drive shafts and gearbox.

Is the UK 6 speed rear diff not a 3.54 unit? Might save a lot of work if it is.

The Spec C diff is 3.9.
The 06-07 WRX STI is 3.54 ratio but is a Torsen diff that I don't want they are not as good as the plated spec c diff.