That'd mean you can be on sit as fast as the 100th fastest head-down skydiver ever recorded...... http://www.speed-skydiving.com/index.php/rankings/eternal-ranking-barometric. If 420 km/h was your normal speed your skydives would take around 30-35 seconds, including time to accelerate and decelerate. I bet they are a bit longer.
I do believe though that you can see that on your app or even protracks, but you are not accounting for burble effects and sudden changes in pressure. For instance, when switching from forward flying (wind on the back while sitflying) to backward flying (wind on the chest). In these cases the sensor will register very rapid changes in pressure, and the formula will result in a calculated vertical speed that is far far away from the reality.