Other than faster melee DX increases the damage of Blazing strike and Shadow.
Honestly I wouldn't recommend pure DX for the reason that your attack speed will be fast enough on its own and Shadow and Blazing Strike won't be in your regular rotation. Just stack element and whack away, also Earthquake Blade and Inferno Blade will be your most usefull skill for mobbing. As for the DX cap, I'm not too sure because I've haven't done any tests, but currently my melee speed is already as fast as I would need and I have bad DX for my lvl.
Search YouTube "trickster online gladiator skill" you'll see my pure ap buff there I only have -7+(-5) DX. (Sorry I cant link videos at the moment as I'm on my phone)