I'm curious as to the benefit of an amalgam reservoir external to the arctube on standard HPS lamps. As far as I can tell, GE has a patent on it and thus is the only manufacturer to use it. According to their literature, the design ensures longer life and more stable colour by allowing only the "precise dose" to be delivered to the arc stream. I used to believe they were the best HPS lamp available, and kept a nice golden-white colour, but now I'm not so sure.

In reality, I have found GE lamps often to suffer colour shifts (they turn "pink"). They do seem to last longer than the Chinese-made lamps (which often have inferior arc tube seals and other manufacturing atrocities) but Osram-Sylvania lamps seem to actually have better colour over life. To my knowledge, the amalgam in Sylvania & Philips lamps have always been in the arc tube itself.

The HPS operate at saturated vapor, so the lamp performance (color, arc voltage, ...) depend on the cold-spot (amalgam reservoir for HPS) temperature.
At first glance, external reservoir allow higher arctube temperature, as it is thermally further away. But what is more important, is the temperature stability. And i doubt, there is too much difference. In both concepts it depend mainly on the power input. And as this power rise with arc voltage, so pressure, there is intrinsic positive feedback and i guess similar on both (the external one being only slower response in time). When this feedback become too strong, the lamp start to cycle. Again no principal difference. The temperature is given mainly by that part of the input power, what is converted to heat on the arctube. And here on both technologies, tube blackening trap more radiation power and convert it to additional heat, effectively rising the tube temperature and causing the positive feedback stronger.

The only difference i would see is in the fact, then the liquid amalgam in the external reservoir does not come into contact with the PCA, so can not cause it's blackening. But i think the PCA manufacturing was perfected by others so, it is way less sensitive toward the blackening from the liquid amalgam, making the external reservoir unnecessary.
So when the external reservoir is not necessary, the seal is easier to made, so naturally less prone to defects, so more finally reliable. And this reliability might easily offset the remaining advantage of the external reservoir, on the bottom line yielding longer lived and more stable lamp.