I've seen HPS lamps that are under a year old lose a lot of light output.

Your best bet is to buy a new lamp. The companies (Lithonia, Utilitech, Cooper, etc.) that make the residential lights don't warrenty the lamps free from defect, so you can't get a new one under warrenty. I'd sudgest Sylvania. Their lamps are very good. Sometimes Mexico lamps act sort of funny, but for the most part, all of their lamps is good stuff.

I know this "tactics" from here:
On most "cheepee" fixtures sold here state in the the Czech version of the manual something like (freely translated) "The lamp included is only intended to check the fixture functionality and is not to be considered as part of the product, so the product warranty does not cover it. The user is advised to install new lamp before using the fixture".
Interesting is, then, the label "proudly" state in English "Lamp included" (but the rest of the description is only in Czech/Slovak/Polish languages).
Then an interesting thing happened, when my co-worker tried to claim the warranty (the cheepee ballast died at lamp EOL): The seller blamed the "lamp" as the cause, advising to claim the damage on lamp's seller. Well, he had to replace it anyway, as my co-worker pointed on the same receipe, then the lamp (replacing the included one, according to the manual) was purchased at the same store, so he finally got not only new fixture, but even new lamp... :-D

Non-banded sodium bulbs are horrendous. One came in a small fixture I bought. The fixture did not have an ignitor-nor did the bulb. The spectrum was near LPS. After a few 100hours, the vacuum failed and the wires oxidised.