If you can control your right foot, the twin turbo should not be too bad (unless it is an auto, in which case it can be fierce). I have a 2000 B4 RSK manual and get about 7km/l in Tokyo traffic using the torque a bit
10km/l on longer trips at hi-velocity, with 12 to 13 possible at more liesurely speeds (your fuel may knock that down a bit).
I like the 3.0 - it is a good motor and the suspension is well matched to the nature of the model. Problem for me is that it is an auto only.
HTH