Ballasts are used where a load does not regulate its own current consumption well enough. These are most often used when an electrical circuit or device presents a negative (differential) resistance to the supply. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the ultimate current to an appropriate level. In this way, the ballast provides for the proper operation of the negative-resistance device by appearing to be a legitimate, stable resistance in the circuit.

A HPS bulb cannot regulate it's own current consumption so the 400 watt bulb will draw more and more (upto 600W provided by the ballast). It'll blow at best and shatter the glass all over your grow room at worst. I don't think it'll effect the ballast but why blow the bulb for the sake of a few minutes light, or however long it takes for the slow start to work up to 450 odd watts?

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