The problem with powerscaling is that everyone wants to scale character 1 to character 2. But tier lists are supposed to be that if characters are in the same tier, both can win. All the talk about mid-diff, high-diff, he wins 7 out of 0 and whatever just should be avoided. If they fight, both can win.
People love inventing a new tier when a character defeats another character, or shows one cool move, but that's not how it's suppose to work.
And if a character is a tier highter, doesn't mean he can suddenly avoid all attack or is immune to a lower tier.
A toddler could hit me in the face and give me a noosebled with a sneak attack, but it doesn't mean I suddenly have to go mid-diff to knock him out.
Roger, Shanks, Mihawk, the old Yonko, Imu, Luffy are all just Top Tier.
Some might look better then the other, because they have hax powers that need to be understand first, and some might have lost because of plot convenience, but Luffy will probably beat Imu this arc regardless.