Well I suppose I could be wrong, but if a digital ballast can power any wattage bulb, then why does the ballast even list a wattage...?
I'll assume your speaking to the fact that the bulb actually turns on. Then No, It's not as efficient as using the proper bulb with the corresponding ballast.
giving a 600w bulb 250 watts does nothing, And while I don't have light measurements on hand to verify, I can only assume it's actually less efficient, and even less of a useful light sprectrum.
bulbs are designed to output a particular light sprectrum for a particular wattage. some bulbs may provide more lumens within a particular wattage, even so. they're still designed to run at the given wattage.