Saying that Sanji is limited by King's strength if he becomes enhanced with the traits of the same race as King is such a funny line of reasoning
Even at face value if one would entertain such reasoning, among the members of a race the gaps in strength can vary to an incredible degree, and you can have Fish-Men as strong as Jack or Fish-Men as shitty as Macro.
Or you can say "Hybrids have to be weaker", but then you have Dellinger who at only 16 years of age was kicking Bellamy's ass, defeated Blue Gilly, Ideo and Suleiman. Basically he runs a gauntlet through Luffy's Grand Fleet, and he's 1 year younger than Luffy was at the beginning of the story.
Enhancing an already powerful human being with incredible potential, with traits of a race that at base makes them much stronger than a regular human, can absolutely lead to superior results when compared to other members of the race used to enhance said human.
Heck, one could argue that races other than humans are kind of lame with their peak progression, when at base level they start with a 10x multiplier. Give someone like Garp those enhancements and he becomes the absolute strongest of the verse