Hi and welcome mate!
The Emerald sure is absolute ace in extended range guitars. It is bright and clear, thus keeping your low strings tight and defined, likely more than you can imagine. Our dear Nolly used to praise the Emerald for modern metal, especially in extended range guitars for a reason. It will also give you a great character as it is between modern and vintage. More open than most modern metal PUs and just more of that old goodness without the drawbacks. Really all you could want I recon.
Thanks for the welcome.
The Emerald had been my first choice, basically for all the reasons you outlined in your post. People suggest the Black Dog as a great choice in ERGs but for me it doesn't have enough output, so the Emerald seems like the best balance of power and clarity.
As for the character of basswood, I'm not really sure. I wouldn't say it was overly harsh sounding but it's certainly much brighter than the mahogany set-neck baritone sixer that I've also got.
I've actually thought of the Rebel Yell before because of it's higher output and more modern character (actually it was described to me as being like an underwound Nailbomb, and I love the sound of the Nailbomb in my friend's seven-string) but I was advised that it might be very trebly and harsh-sounding in basswood with a bolt-on, since it's been designed for LP-style guitars which are obviously much darker.
I don't mind a bit of high-end for clarity but I wouldn't want anything icepick-y.
As to the neck pickup, ultimately it's my intention to replace it as well, but right now I can't afford two BKPs. In addition I basically just use the neck for crunchy semi-cleans with the volume rolled back over halfway. I seldom play with the neck volume on full so balance isn't too much of a concern, also I think a BKP might be wasted there with the way I play.
I use dynamics a lot to get different tones, using volume pots and switching positions to go between clean, crunch and solid distortion, so I actually favour mismatched neck and bridge pups for more contrast.
I would avoid Warpigs in an 8-string.
You would be better off with a brighter, tighter pickup like an Emerald or Rebel Yell
I also know someone who is happy with Aftermaths in his eight string.
Aftermaths look interesting but I prefer alnico magnets over ceramic. I also think I'd prefer something more organic and less compressed.
Could you elaborate on why the Waarpig might not be a good choice?
I don't play (or even like) djent so I'm not after a tone like that at all, which is annoying just because a lot of the demos I've seen of people using eight-string BKPs seem to be aimed towards that sort of style.
I'm not even a metal player so I don't need loads of distortion, just something higher than the bridge enough to slam the preamp harder.
I prefer a more grainy, growly character in the mids which is why I'd been recommended the Warpig. It didn't seem like the most intuitive choice to me either but I'm trying to get ideas on it.
And does anyone have opinions or experience of the Holy Diver?
The description on the site makes it seem like a great choice, fat mids, warm bass and rich highs, plus it's designed with brighter guitars in mind. Seems to sound a bit like a Seymour Duncan JB which is a pickup that I quite like.
This is an example of my playing and tone, if that helps at all.
Apologies for the wall of text, I'm just trying to get in as much information as possible. As a broke student I can only afford one BKP and if I make the wrong choice I'll be stuck with it, which I'm really keen not to happen.