I'm looking for some headphones that could let me distinguish at least some of what is going on in this song. I currently own a pair of Hifiman RE 400 and can vaguely hear what the guitars are playing.
Warning: loud chaos ahead.
Speed and mid clarity is what you need... do you have a budget in mind? Grados sound pretty fast (325, 80), the Hifiman HE-400 is a quick planar model, perhaps some AKG models... those are options up to the $300 range. It really depends on your budget. Also, you could do a search for 'fastest headphones' or something to get some views. I've heard none of those btw, I'm just trying to get you some momentum in your search.
Well, recording and mastering quality is often more important than headphones or speakers. Good recordings with high dynamic range will bring you the clarity you are looking for (and more). Unluckily, there are no many high quality metal recordings. You could also read about the so called Loudness War.
What I mean is, you can spend thousands to get one of the fastest headphones on Earth, but compressed music won't sound as clear as you might wish after all, in fact many fast High End headphones don't sound good with modern/compressed recordings. You can read about Sennheiser HD800, you'll find many people saying that they only shine with jazz, classical and acoustic music.
Maybe you should try a pair of Grados.
What sources do you play the audio from/
What DAC and headphone amplifier do you use?