T5s can reach 114lm/w for a 35w H8. Now as far as I know, no T8 or T12s hit this. T12s hit a max of 93-4lm/w, T8s 86-95lm/w. I am not endorsing T5s, just a thought though.

The point in higher Lm/W is to be greener ie take up less resources and pollution when running on polluting fuels. This will be the case if the T5's and their ballasts are reliable enough that their manufacture / shipping / replacement etc. dont outweight this, and when used for the purposes they are best for and not anywhere

T5 do reach 114lm/W, but only inside covered fixtures and HF ballasts. Unlike T12, you can not use them as bare tubes, what mean you have to count for losses in the optics (refractors,...), so end up in 90..100lm/W.
Partly because they are too intensive (too small surface for the light output, so too glary) and partly because they won't reach their optimum temperature, so their efficacy would drop down to the 80..90lm/W range.
And the 100lm/W they reach only after warmup, for immediate light they reach barely 50..60lm/W (before the warm up), what is way worse then T12, so by far not usable for intermittent use (when immediate light is specified, like on stairways, the extra output after warmup is then only overillumination). You may help yourself by the boosted output ballast, but for the T5 you would need power boost about 200% for the nominal light output, what stress the electrodes a lot, so lamp life suffer. And such ballast would need some sort of feedback from the lamp to prevent the ballast forcing 70W into already hot lamp (e.g. after few second power dip sufficient to reset the controller)

The F36T8 reach ~103lm/W on HF (3300lm/32W power input), while before warmup it s nearly 80lm/W (so boost to 36W, no harm at all), the T12 with triphosphor would be about the same 80lm/W since ignition and even on colder temperatures (so standard, simple programmed start would be enough).

The reason why i operate my T5 21W fixture without its cover, is because the cover is white and not transparent, so it hidden the lamp and i want to see it.
And also the fixture is so high, that i can only reach it with a ladder, and this i can do only with the maintenance person.
I see that i have no option for more efficient replacement for my HF T5 21W fixture, other than a HF 18W T8 fixture, for efficiency and flicker free operation.
For magnetic ballast, the only option is two 18W T8s series with a 36W T8 ballast to reduce flickering, while allow me to replace the glow starters with flicker free starting pulstarters. But of cousre, with triphosphors, my room will be overlighted, unless i will install an energy saver T8, which have the same intensity as the halophosphors 18W T8, but with less energy and still better light quality.
With two PL-S magnetic fixture, i have no control on the starters of the lamps, as they are build into the lamps, and there are no PL with electronic starters (Except the PL-L, which are operates with an external starter, and they have no efficiency advantages over T8s).
I highly recommend to operate energy saving CFLs base down.
My Osram Duluxstar 8W/827 in my spot, suvived for many years, only because the fixture points up, so the lamp operates base down, such as the heat of the tube heats much less the ballast.
MY Hyundai TEVA 20W 827 helical CFL, survived more than a year now, and operates in my table lamp, without its closed lampshade, base down and also connected to a surge protector. My Philips Torando 23W/865 also had long life because of this.

Using the fixture without cover have the additional advantage of lower temperature in the fixture, which mean longer life to the HF ballast

The fixture being up high so you can't reach it is not an advantage - Even without the cover, the fixture does not have any exposed live parts which can be _unintentionally_ touched

Any PL has very low flicker since it is bent and both ends are next to each other, eliminating allmost all 50 HZ flicker

Since PL have lower arc voltage than 2x18W fluorescents, i think that it would have less 100 HZ flicker as well. Also if i am correct at this assumption, runnng 2 18W T8's on 2 18W ballasts would make less flicker than on 1 36W ballast, though be less efficient. Then just install them side by side wired in opposite directions

PL-C and PL-D have 4 pin versions for use on emergency ixtures with external glow starter. They can be used with any suitable magnetic ballast and starter as well as HF, but are very expensive (about 50 NIS / lamp)

I did hack a PL 9W with stuck starter (result of being left in series with EOL PL on the same ballast and flashing) by opening it and replacing the starter with ordinary S10 (use S2 if you want to use in series with another PL) You can as well put in a PCB from any electronic starter

A 2D is yet another good solution, but unfortunately 2D lamps are not widely available in Israel (though there are Eurolux ones for 30 NIS if you know where to ask, and Eurolux are usually decent). Good 2D fixtures from the past that existed in Israel are Coughtrie CPS and knock-offs of Coughtrie products made by Gaash