Can somebody please help me ID my turbo and diff models? Where to look (not generally but specifically) and what should I look for??

Thanks

 

On the turbo facing the front you will see a flat milled small area. This contains all the turbo data. With the aid of a torch along with the other mumbo jumbo you should be able to see TDO4, TDO5, VF24, VF28 or whatever. You will need to remove the heat shield to see this. Alternatively if you still have the OE turbo it can be identified from your exact model type year vehicle number on the chassis plate etc.
To identify the diff, I assume it is still in the car, so a starting point is exactly as above, checking that the diff matches the anticipated spec for that model. You could also accurately check your speeds at 3500rpm in each gear and then compare that information with published information for each gear box and diff.
If the diff is out of the car then it is very easy to check your rear diff. Simply count the crown wheel teeth and divide by the pinion teeth. For instance 37 crown wheel teeth and 9 pinion teeth = 4.111 diffs. You will also find it easy to identify the rear limited slip diff unit to verify you do have a rear LSD.
Impreza diffs are generally 4.444, 4.111, or 3.9 but a UK car has a 3.545 rear diff with a 3.9 diff which is converted to 3.545 via a 1.1:1 dropper in the gearbox. Jap gearboxes are 1:1.
I have come across at least three cars lately with differing front and rear axle ratios. As the District Judge informed Mr Peter Croney recently "You do not have to be a physicist to work out that the front and rear axles have to have the same overall ratio".