Agreed, turbo performance in game is realistic (boost builds based on throttle position and torque at the crankshaft) but the acoustics leave a lot to be desired. By no means am I a turbo expert but I have broken the acoustics into 2 main categories I think would make the most difference.
I. BOV sound.
The BOV behaves kind of like an on-off switch in game like you said which is not how it really works (air diverters in modern cars like the FoST/FiST don't count).The higher the pressure inside the charge pipes, the louder and more dramatic the BOV should sound when the throttle snaps shut and the engine goes abruptly back into vacuum.
Often times revving a car at idle will not even trigger a BOV noise at all, as there isn't enough load on the crankshaft when the tires aren't pushing against the pavement.
When looking at the telemetry, the game physics seem to have these concepts all covered but it would be nice if the BOV sound was dynamic enough to give audible feedback about that information.
II. Spool sound.
It would be nice to hear some kind of feedback from the turbo(s) as they spin up and come into boost, see clip below (~1min, no music)