Ok so did a big more digging, referencing the MY09 US Legacy brouchure.
Now these are kerb weights (auto only to give like for like on all models, as 2.5i Limited was auto only)
*Note: Manual vs. Auto transmission = 25kg's only (auto heavier)
2.5i Limited (auto) = 1536 kg's (3388 lbs)
2.5 GT = (auto) = 1588 kg's (3503 lbs)
So as close to like for like, is the 2.5i Limited vs 2.5 GT, as both are spec'd up about the same interior and drivetrain etc.
This shows there is only 52kg's in it between N/A and Turbo.
Only real difference is brakes... everything else is the same...wheels, steering rack, suspension, etc).
Brake discs = 12" Front vs 11.5" Front (GT vs. 2.5i), so maybe that is 5-10kg's per brake rotor?
So based on this, I call the 2.5GT vs 2.5i engine weight issue BUSTED
