Cold Take: PK Luffy will be much stronger than PK Roger, they wont be relative
Roger was super strong but he never had the same competition that Luffy has had. Luffy already defeated Kaidou 1vs1 in Wano, In Elbaph his enemy is Imu himself who is much stronger than Roger ever was and there is still Blackbeard who will problaby be as strong as Davy Jones if not stronger when Luffy has to defeat him.
Roger in comparison never had that level of opponents, he didnt beat Big Mom for the Poneglyphs, Whitebeard was basically his friend so their fights were never serious, Garp was an enemy but Roged never beat bim and Shiki was saved by an storm. So if you look at Roged he didnt beat his rivals like Luffy will have to do and his rivals were much stronger
And I havent even mentioned other possible opponents for Luffy like Akainu, Shamrock or the Man Marked by Flames. The amount of competition Luffy will have to beat to become PK is much higher
Kaidou> Shiki
Elbaph Imu>>>> Prime Garp
Blackbeard>>> Prime Whitebeard
Man Markes by Flames> Big Mom
Akainu> Prime Sengoku
PK Luffy> PK Roger Mid-Diff