Working on the generally accepted notion of 10bhp per PSI boost increase, a full BPU car at 1.2 bar = increase of approx 6.5 psi, which would equate to 320+65 = 385 bhp.
Most BPUs don't run 1.2 Bar, more like between 1.0 and 1.1, so as Tricky-Ricky says, average is probaly more like 365.
All this is pointless however, as you can't measure flywheel HP, so an accurate RR session (eg Surrey RR) giving the rear wheel HP is a more meaningful comparison, providing you have results before and after any mods on your car, as no two cars are ever the same.