I dont think Sanji's kicks are stronger.
F = Mass x Acceleration
So lets assume that Sanjis Kicks are mass of 3 and his Acceleration will be 1 while Luffys mass will be 2 andd his acceleration will be 3
(I think Luffy's acceleration is understated by alot with this example, but its just and example, also we know for a fact that luffy kicks faster the sanji (jet stamp) so we use sanjis speed as a base for luffy, so in this current example we are saying that luffys kicks are 3x faster then sanjis (which i think is underrated))
So Sanjis F = 3 x 1 = 3
While Luffy's F = 2 x 3 = 6
know we can argue who has more mass and what number should be used for acceleration, but i think luffy is 3-4x faster then sanji atleast and if he goes gear 2 then i think its 6-8x faster then sanji.