Hey rabidgerry, this kind of playing changes the production paradigm quite a bit. The notion of it's more important what you don't play (all at the same time preferably LOL) to leave those minute spaces you brain needs to stay sane and really enjoy music is being turned on its head (not that I agree or disagree, each to their own).
Anecdote: A friend rented out his PA/lights etc for a Thrash/Hardcore Screamo gig and asked me to mix (the first of 4 bands). I didn't know what sort of music it was, just said sure... So I mixed the first band, you could at least hear everything but I wasn't sure I'd done a decent mix (until later). My mates son usually mixes but had to bugger off, before he left, when I was doing drum sounds, he said turn kick eq +16db @ 4k, (normally I'd add a "little" 2k for kick), I asked why, he said otherwise you wont here it (so as you say extreme). The next 3 bands another guy mixed. I hadn't bothered with the drum noise gates, first thing he did was insert them and set them really aggressively (I now know why). But even with all that it was just noise. The last band were Red Shore (big here in that scene apparently), someone said the guitarist were really good in this band, well went down to the stage and watched their fingers, looked good, could'nt hear it though...