i read in the mid 90s that traditional fluorescent and other gas discharge technologies have gotten close to their inherent limits. traditional fluorescent lamps have higher stokes losses in the conversion from uv to visible light than in white leds using blue leds and phosphor. the mid 90s was when the lighting industry have started research into new lighting technologies to replace traditional incandescent, fluorescent and hid sources. i remember research in induction, sulphur, etc and it was also around the time that bright blue leds came out which paved the way to white leds.

however some certain types of plasma could in theory come close in efficiency to leds, such as sulphur. in theory, sulphur could reach 70% conversion into light, largely because it does not generate uv. however it has a very strong chemcial reaction that no known materials could be used for electrodes. so they could only be made in high wattages and run by microwaves. so therefore they have not become practical for most lighting needs. basically with leds the big advantage is not only direct conversion to light, they can also be precisely made to generate specific wavelengths without uv and ir.

The sulfur require high operating temperature and an induction feed. This combination limit them for the microwave supply only, but there is the problem, than generating the microwaves mean an efficiency only around 50% and coupling it into the discharge mean half of the remaining 50% being losses too (either poor coupling, or the coupling cage block the light output), so the overall efficiency won't go above 15..20%. The high lumen output then go on account of majority of the light energy being in green, where you don't need as much watts for lumen.
On top of that the hot components (the arctube) would always have significant thermal losses, so the arctube efficiency in generating light from microwaves won't rise further.
Moreover such contraption is complex, require moving parts (fan, arctube rotation,...), so it would cost would remain high...
With LED's the present efficiency into white is around 30% (when omitting red it would rise slightly, but not as much), so when engineered for the maximum efficacy and throwing the CRI away, it could go way above 120lm/W (missing red, so it would resemble the light of clear MV's or even the medium pressure ones)