babygro, I don't keep an HPS tube long enough to see much degradation of the output. They get changed out every 12 mos per (in my case) GE's recommendation for LucaGrow HPS tubes. I could certainly do the experiment you suggest, but it's bit moot.
Regardless, the output measurements one gets with a magnetic ballast will be identical to that obtained with any electronic ballast. Ballasts are just current limiters, no matter whether that limiting is accomplished with an inductor or a PWM power supply. Electronic ballasts, just like magnetics, ONLY supply current at a certain limit- no more, no less, no magic involved.
Electronic ballasts start the tube a bit more 'softly' but that doesn't appreciably affect the luminous output, certainly not over the course of 12 months service life for HPS tubes as is recommended by most horticultural HPS makers. Softer starting may give more starts per tube life, which means something to a council which runs a few thousand streetlights and needs them to last several years.
buster69 over on OSA inquired of Philips lighting about electronic ballasts and got this reply:
Many thanks for receipt of your enquiry.
There is no output improvement by using EL (electronic) gear for SON lamps. The reason of using EL gear for SON gear in Horticultural is energy saving at gear side and high reliability (EM (electromagnetic) ignitor and capacitor are easily faulted under high temperatures).
Kind regards,
Richard Shepherd
Product Manager - Professional
If you keep your magnetic ballasts cool, they should last approximately forever. Given that most horticultural lighting systems have remote mounted ballasts, well away from the heat of the HPS tube itself, they do tend to last a very long time.
I have a 400HPS mag ballast which I got 2nd hand, known to be at least 10 yrs old and still in 24/7 service and also a couple of 1000W CW HPS ballasts which are 7 years old, presently in 12/12 service.