"...since what matters is the essence" is a dangerous forum path; this could certainly lead to "Was Wagner the first metalhead?"
Does appearance constitute content, partially?
And does appearance express content?
Viscerally, do we want to see Slayer with nice haircuts and khaki trousers?
Regarding good hip-hop: is it possible that good 1st grade writing could be made, that is at the same level as good 11th grade writing?
We can probably agree on the essence(s) of Metal>>>Romanticism, vir, power-worship, meditation on death/impermanence, beauty of horror, etc.
Hip-hop that takes street-life horror as it's subject may approach those. MC Eiht, Scarface, Clipse, and KRS-One come to mind. Yet these remain strictly personalized, logical narratives.
Musically, hip-hop will doubtfully ever deliver what I (we?) find good in Metal: HEAVINESS, long phrasal structures, delightfully shocking and intellectually stimulating riffs and techniques. Hip-hop will not approach the cosmic, transpersonal, or post-human.
Most gangsta rap is a better/more poetic expression of anger and inner-city rage than Pantera.