Lucci can't enhance his DF any stronger since it's just a simple Zoan and he's mastered all three forms of himself. Plus he's mastered Rokushiki, meaning he can't get any stronger with that. So what does that mean? It means Lucci was at the pinnacle of his strength when he fought Luffy (for the last time).
Now, since we know Lucci can't get any stronger without Haki, let's compare both him and current Law (without Haki): Law obviously wins (Mid DIFF). So now add Haki: Law still wins (Mid DIFF).
Furthermore, in order for Lucci to close the gap, he'd have to have - not posses the basics of Haki - but posses high-level proficiency with it. And Rayleigh himself (a Haki phenom) said no one is capable of using Haki like Luffy could, in just the 2 years he had for training it. And the only reason he did so is because he had the enormous drive, determination and pure willpower to never lose anyone ever again. And even the level to which Luffy got his Haki to, isn't that strong (as evident when Hody bit Luffy and Luffy said something along those very lines).
Hence there's no way for Lucci to be able to make that much of a leap, to be able to tango with the likes of Law.
