From what I understand, the SLIs tubes were ‘pressed’ out to get the shape, but Thorn had a very high volume of scrap mouldings during the process?
SOX was the last design before the LPS lamp went obsolete?, who knows?, if production would have carried on to the next, more efficient level, the may have changed name to something else?
There wasn't anywhere to go. If you go through all the generations, the efficacy of any given lumen package or power rating was just converging towards a kind of ultimate limit. And the SOX were practically few percent from that limit, mainly due to the need for high temperature operation (so big part of the power is consumed by just keeping the thing at operating temperature).
Every light source technology follows similar path: Early on the efficacy grows massively, but then it almost saturates. Only rarely there is a steeper proggress, but even then it means quite a departure from the original.
Incandescents started few lm/W, then in the first half of the 20'th century saturated at the almost present 10..20lm/W (depends on the power and voltage ratings). Only active chemistry (halogens) were able to boost it by about 20..30% but that was already at the cost of quite some drawbacks (cycling sensitivity,...).
Same like MV's stopped at about 55lm/W, but were at 50lm/W already in the 50's.
Except fluorescents, pretty all discharges face problem with the energy required to keep the thing at operating temperature. LPS suffer from large size, high pressure lamps from way hugher temperature, HPS being the lowest, hence the highest efficacy. But pulse MH were able to catch, gaining in smaller size burner.
Similar was development of the optical systems, it saturated at practically uniform illumination with 95% efficiency with a small size light source. Not much further to go...
White LEDs started fractions lm/W, then it was big "wow" when they met the efficacy of incandescents, today they are at almost 50% efficiency (150lm/W of a CRI80+ white light), yet still there is belif in the industry it should be possible to go a bit higher (it is a cold running device, so no losses to maintain the heat). There is still "hanging in the air" the possibility to reach that efficiency somewhere near peak eye sensitivity, then it would mean more than 270lm/W (50% efficiency for a monochromatic green). But no major breakthrough (like doubling it,...) is to be expected either - just because it can not be a perpetual motion device.
