Gaban's feat is Oda's MO of having respected characters go out with a bang. He gives flowers to characters who go beyond their limits with a significant power gap/disadvantage (WB two-shotting Akainu , Zoro permascarring Kaido with Asura, Ray/Gaban 2v1ing Oden, Law puncture willing BM)
Does this elevate Gaban's portrayal? Yes, he's not mid-diff material for any top tier, may even go extreme diff with some admirals
Does it change the hierarchy? No, we already have confirmation old legends are beneath current yonko (Bumleigh's admission)
Does it downgrade Imu? No, cuz he was distracted and already nerfed. The attack ultimately did little to him
Oda has a clear hierarchy in his mind of where chars like Runleigh and Waban lie. He is relative to yonkos in their prime but still below them (difference is debatable), they always matched up with other lower subordinates. Oda wouldn't even let Runleigh fight Primebeard to save Shakky despite it being his spotlight. Gaban fought off other weaker Rocks members instead of Shiki, BM, Primebeard
Almost certain Gaban won't be above Shanks in terms of haki prowess. Despite the crazy feat, it's said his talent was still below Shanks, who could genuinely copy Roger's own techniques. It's ridiculous given Kamudo feat is arguably top 3 haki feat but Oda made this clear already