I don't doubt you when you say that it sounds better as a wav, but if you planned on submitting this for a NG competition and you know it sounds like this as an mp3, why not try to remaster it based on the limitations/characteristics of the mp3 format? That's my main complaint and suggestion.
I also agree with Slayer about spicing it up with extra riffs, and the kind of mixing changes to make. Other than that, I'm no expert in this genre. This is all just audio design tips.
Also, it depends on what kind of speakers/headphones one listens with. If you have something super high-end or unique that not everyone else has, you would get a very different experience from what most people get. So maybe it sounds perfect to you, but super muddy to me.