The civillian packets are always moving at 1000 km/s and they vary in size from 40 tonnes to 250 tonnes. I will have a harder look as I have packets of <1 tonne, 3 tonnes, 9 tonnes and 19 tonnes from my colonies. All launched by a single mass driver. I can say that the 9 and 19 tonne packets seem to have velocities nearly a factor of 2 appart. I don't think in any case I have saturated the capacity of the launcher since I produce around a maximum of 1200 tonnes per year (150 tonnes per year * 8 minerals)
Also each CMC comes with a mass driver so in principle the civillian packets are being launched by upwards of 20 mass drivers.
added in edit: Ok looking at the velocities and such they seem to scale reasonably well, within anyway rounding errors for the packet size I'd say. So the packet that is "0" is moving at 93K km/s, 9 is moving at 6500 km/s and 20 is moving at 3500 km/s. However, civillian packets all move at 1000 km/s...regardless of size. However no mass driver is saturated at this point.