Dear Lamborghinisti,
You should have very good flow through the vents.
The vent diverter system is controlled by vacume servos. I had to go throught the whole system in my 97.5 roadster as the controls on the dash wouldn't work right.
My problem was vacume was not getting to the dash controls. In my case it was the line to the vacume services tank had split and come off while a check valve was installed the wrong way ( yeah....guess how long that took to find.
)
The vacume services tank is located behind the coolant expansion tank.
If you are getting high speed flow only through the defroster vents, your system is in default and not getting vacume.
Shamile