I read here that the S55 may be running TF035 turbos
http://f80.bimmerpost.com/forums/sho...d.php?t=924954
Subarus have been using a single TF035 on the Forester and after some googling I found several forum posts saying that people boost them at 12-15psi for years without problems. Also Subarus are using the 14T sized compressor wheel while the S55 turbos have the 17T size, according to the link above.
Therefore it seems possible that two TF035-17T turbos could boost up to 24psi which would result in significant performance gains.