Good to know!
Well, fwiw I replaced one bulb recently (the burnt one in the image above) and it doesn't seem noticeably brighter than any of the other ones in *any* of the fixtures.The mystery is why they aren't very bright. Correctly working ballasts and new T12 HO lamps are VERY bright, 6000+ lumens per bulb.
What would be the benefit of doing that? TBH, I *hate* working with 8 ft bulbs, start to finish, from getting them home from the store (have to make a special trip with the truck because they won't reasonably fit in my car, but 4 footers do) to disposal. I don't live in a metro area; some things that are commonly available elsewhere are... not so much, here. If I have to get up there to pull apart and rewire the fixture, why would I do that instead of just pull them down and replace them with LED fixtures of a known history?You could swap out the tombstones for single pin and use 8' LED. This works well as you can rotate the bulbs out to cover a wider angle and help make up for some of the LED issue with too narrow output. I wouldn't use these any lower than the 9' you have as they can be too glaring.