Here is some info which may help some..
Some research I did suggests that the R180 in an '07 Sti is heavier than the R180 in an '06 Sti, possibly adding as much as 20 lb.
This is because the '07 has a torsen limited slip locker, while the '06 has a mechanical, clutch-style locker.
Since the weight difference is rotating mass, it's effects may be equivalent to more than 20lb of weight savings.
A theory I have, which is not verified yet, is that the '06 and '07 6 speed transmissions are heavier than the '04-'05 by quite an amount.
I don't really know what it is, I am thinking it may be 40 lb.
This has to do with the center differential. The '06-'07 have an additional torsen locker, again rotating mass.
FIY, for the JDM cars, the years and nature of diff changes were the same as US. However, for the Spec C, they never went to rear Torsen, although they did so for Sti. I never looked into the center torsen for spec C, but I can look into it.
If you're looking for reliable, low mileage swaps, the JDM swap places usually have low mileage items. However the JDM 6 speed have shorter 5th and 6th.
They always have the same final ratios as the same year USDM Sti.