Cannot find it so here goes.(this will be very around the bush but it will all make sense in the end)

Wrc engines run very high compression and very small restrictors. 34mm to be exact. This obviously restricts the engine to 300-320bhp.

Due to the restrictor at low rpm you can run big boost at high rpm you cannot. The turbo sucks air through the restrictor. And at a certain point the restrictor will not let any more air through but the turbo keeps trying to suck air through, this causes a depression after the restrictor. You have to carefully monitor and measure the depression as the turbo can over speed and break.

A typical wrc engine boost curve will be at 3000rpm 36/38psi and at 7000rpm 7/8psi (can change dependant on depression) you run very high compression as basically as you rev the engine you turn it back into an atmo engine (sounds strange but it's true) boost goes so you make the engine produce the power, not the turbo. Big cams also help with this, but timed not only for top end power, as a rally engines most usable power range is mid really 3500-6000rpm (where the 300bhp is!)

Hmmm I think that's it, I will read through and correct anything in a sec and add a couple of graphs shortly