If the engine flows too much air for the turbo, it will choke, limiting achievable power and rpm levels required.
Same way as a 2.5-3" exhaust is fine for a stock motor, but not one with a bigger turbo/motor.
PJ has got the rated number and more out of a few PTE turbos, 2l, 4cylinder, 40+psi.
Anyone care to work out the equivalent flow of a 3.2l, 6 cylinder at half the boost?