There's a digital ballast maker (Lumatek) which is claiming up to 30% greater luminous output from a 600HPS, which, if true, would put a 600 within cooee of the output of a 1000HPS.

I had a big ol' dummyspit with some clods on another board about this; I called bollocks on the greater luminous output claim. I reckon bejay's right- the luminous output comes from dropping watts across the arc tube. If you want more luminous output, the ballast would have to allow more current through the tube- in excess of its ratings. Not a happy thing for tube life.

Digitals start up the tube a bit more softly, so you'll get more starts per tube life. However, if you replace your HPS tubes annually as most makers recommend due to output degradation, it's a moot point. An HPS tube has plenty of starts left in it after 12 mos in service, even on a plain ol magnetic. Now, if you're a city council running a bunch of streetlights, it'd be a good thing to get another year or two out of a tube. Streetlighting can tolerate much more output degradation than can horticultural HPS users.

Digitals are more efficient in regulating current as they have no iron-cored inductor with the inherent eddy-current heat wastage. The savings on a 600 HPS on a mag compared to a 600 on digital is about 55 watts, ~9% overall power reduction.

So, is it worth converting a 150 to a digital? Probably not.