I have the carbonetic 1.5 front and 1.5 rear lsd and the 05 STi 6spd gearbox.
The gearbox alone was a huge upgrade over the 5spd.
The 05 has the 35/65 split. Where the 06+ had the 50/50
For the street and just playing around the 35/65 is pretty fun and can be adjusted with the dccd controller. On the track I have locked the dccd to 50/50 and with the diffs I was able to knock 2 seconds off my lap time. The nice part of this set up is the amount of grip I have. And the fact that I can let off a tad and feel the tires dig back in after loosing grip mid corner. (70mph 4 wheel drifts can be hairy)
In the future if I buy another Subaru replacing both diffs will be the 1st upgrade.
The oem front lsd even in the 6spd is crap. The only drawback is if you are going to drive it everyday. They can be louder, harder to steer more vibration. But my car was not stock to start with.