Hi,
The bulb and ballast must match for proper operation unless the ballast is switchable. A 150W ballast will most likely not fire a larger wattage bulb and even if it fired it somehow, the ballast will certainly not put out the higher current required. Another point is the 150W DE bulb sockets are different than 250W DE bulb sockets. Not sure where you would have read otherwise about the bulb/ballast.

Cheers,