Yep, turbo brushes usually have a true relief valve, in the form of either a slider or rotary vent. Allows you to have enough airflow going through the turbine to drive the brushes at a sufficient speed, while relieving some of the suction so the head doesn't bog down into the carpet.
On the rare occasions I use the turbo head on my Sebo D2, the vent usually needs to be left open. Running the cleaner at a high enough speed to drive the brushes usually results in the head sticking itself down onto the carpet. Slowing the motor down would prevent that, but also makes the turbo head less effective. Flicking open the vent solves the issue without affecting performance.
With regard to higher suction filling bags more quickly, I think after a certain level you reach the point of diminishing returns. There is only so much dirt that can be removed from any carpet by vacuuming. I've had Hoover cylinders with 700-800 watts that filled their bags at a rate of knots, and a 600 watt Hitachi upright that put many modern cleaners to shame. Now we have cleaners with twice or thrice the wattage, they don't fill *that* much quicker.
