If we are retro fitting an existing fluorescent then we always just by pass the ballast and install led bulbs. I see nothing wrong with that. They work great.
The fluorescent reflector is about 70% efficient and you lose some through the diffuser too. That’s why the high efficiency T5-HO stuff has almost a mirror reflector and no lense at all. It is competitive with LED fixtures. You lose the same lumens (about 30%) retrofitting with LEDs.
If you go with an LED fixture (not retrofit) you lose 3-5% through the lenses, about the same as the fluorescent diffuser, but that is it plus you keep all the “fringe” light in the area it is aimed at. So you get about 50% more usable light per Watt. That’s huge.
Also if you go for 3500k instead of fluorescent 4500k the light is closer to optimal for human eyes (natural sunlight, 3500k) which makes everything more visible even if the foot candles say otherwise. This is light efficacy, good for another 20-25% although as I said the FC meter doesn’t measure lighting efficacy.
On a lighting efficiency point of view retrofitting is not more efficient except maybe on old crappy fixtures that are better off replaced. If efficiency is the goal, replace. But if longevity is the goal then it should be in favor of LED retrofits but the issue is that corncobs are not well built. They claim 10 year life but frequently only make 3-5. I’m not sure but ventilation in a troffer may be non-optimal. LEDs must stay room temperature or less or they fail very quickly.