I'd love to see subtle condensation effects around the wheels of the cars, and even a shock cone effect at transonic speeds.
When the Norvos track was added, the snow/dust cloud effect around the wheels sometimes looks just like the effect I'm talking about. Keep it subtle, but noticeable and I think it would add alot to the impression of violent speed we already have.
With the treads on those wheels, their RPM, and the speeds the cars travel, the air around those wheels should be incredibly turbulent, so condensation effects around them is totally justifiable.
As for the shock cone, it's something I'd want to only see RARELY, as in ~1000kmh+. A shock cone flashing around the car for a few milliseconds would add a serious "WOW!" factor, to ice the cake of achieving a speed that high.