FYI if anyone else is looking for this specific answer: I was finally able to call Pentair customer support at 800-831-7133 for technical assistance on this question. The tech assured me that the housings for the Amerlite with the 300W, 400W and 500W bulbs are identical and that a 500W bulb can be used without issues. The original pool store guy and an employee from another pool store that I also asked who said it was OK were correct.
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