Most fixtures are designed around the lamp/bulb that they already use. Anything more powerful (more wattage) can negatively impact a fixture (ie: blow fuses, melt components, overheat the unit, etc). The increased power draw will exceed the limits engineered into the fixture.
I could only recommend trying different manufacturers of the 250 watt lamps the fixtures use. I would not try a lamp with an increased wattage over 250 watts for the reasons listed above.