No one is denying that Superman is above various Tier 5 to tier 4 feats in later comics where Superman actually does do some of those feats through his pure physical strength recieved from the yellow sun. But even Green Lanterns are individually not reliable source of scaling. they can do some Tier 5 to tier 4 feats in some issues where they have strong will power. But likewise, there are issues where they have depowered empathy and/or author portrays them as also being Tier 6 and below. And as far as I'm aware based on what Firestorm and FanofRPGs were saying, Green Lanterns showing Tier 5 and above levels of power and still getting overpowered by Kryptonians doesn't quite happen in the same comic pre Death of Superman. Comics where the latter happens were the same comics where Green Lanterns were also portrayed as weaker than usual. But other showings where Tier 5 stuff did happen have consistently portrayed as Green Lantern being the strongest JLA related characters by far, or the fact that Superman himself actually did got a lot stronger than he normally is upon using a Green Lantern ring.
Also, I think we should avoid an absolute lowest limit. There have been sayings it's possible for Superman to literally get depowered all the way up to Street level. And also, I disagree with saying "Without yellow sunlight" as yellow sunlight is the only reason he got higher than Batman level in the first place. He maintains strength as long as he doesn't overuse Krptonian powers, and gets stronger the more he sun bathes, with acceleration also growing stronger the close her gets to the sun's core. If his pre Death of Superman key needs a Tier 5 end, it could be with a Green Lantern ring I wouldn't mind though.