I believe Oda can make either one win if he has a narrative for it
i don't give a shit about power scaling One Piece anymore, cause it doesn't really follow logic tbh, it's more plot based
so if Oda wanted Shiryu to kill / defeat Kuzan, he could 100% do that by making him hide in the shadows with his invisibility and stab him behind his back with some weird Haki or some shit, he also has red evil demon eyes like Zoro did from Pre-TS, so maybe Shiryu also has a "Ashura" of his own
i really don't care anymore
when Oda makes up his mind which one he wants to be stronger, then we will see