I've looked at all the "popular" sites that Google shows (like zimlab and jeffludwig) for mods, but I'm wondering if there's an experience point mod. I don't need anything really crazy, I'd just like to get 30 levels or so before endgame, and with the default xp it seems unlikely that I'll get more than maybe 20 for the more expensive classes(like my poor ninja/bishop), given my party size (six plus Vi).

Does such a mod exist, or am I better off just getting the Cosmic Forge editor and playing around with it?

I'd imagine a mod like that to be mostly invisible. As it would be so small and such a niche thing there's no way it would be all too popular... so no: I've never seen or heard of such a mod. Of course you could limit your party, possibly temporarily (with dead members not receiving XP), and/or grind XP. Besides that there's the Cosmic Forge.

Not sure what you want it to do. Modify the experience every single monster gives by a small amount? Put a fixed encounter into some corner, to meet the "super xp gain Poison Seeker"? You could also lower XP requirements for Bishop/Ninja. I've got no clue about the CF mind. It's only that it surely offers a trillion ways to deal with this. If just any solution is all you want there's surely some mod somewhere that works, likely changing a bunch of other things as well.

Personally I'd always seek the challenge in that. Find out how to grind a Ninja/Bishop combo, kill the rest and go for it!
I'd imagine killing off swarms of dangerous weaklings with AoE spells might be nice for mid game. Later on straight out farming Rapax with a Cane swinging Superman should work. I grinded a lot of different skills but never XP so I don't really know. Finding out how to grind my stuff has always been fun for me. It's pretty dominant in Wizardry whenever push comes to shove. Check XP tables and enter the "speed kill" challenge. Or find encounters with meaningful xp that can be won without health loss while not giving commands and let your machine run through the night...
I heard you: you want a mod. Nonetheless that's what I'd do. There's surely a way that needs nothing but what you've got already.

And that's not bad advice, except that - and you would have no way to know this - my party is kind of crazy and I have five bishops so grinding them doesn't really save me much time, ya know?

Valkyrie (x) + Ninja (1) + Bishop (8)
Samurai (x) + Ninja (1) + Bishop (8)
Ninja (x) + Bishop (11)
Gadgeteer (x) + Ranger (1) + Thief (1) + Alchemist (5)
Bishop (x) + Monk (1) + Bard (3)
Bishop (x) + Monk (1) + Bard (3)
Vi Domina

It's not a huge deal. I've been off and on for months in this game and I'm about clevel 15 and have only finished Trynton and most of the Swamps. At this rate I won't need a mod to get to clevel 30, it'll just happen around 2018 when I finally get to the Rift XD

That's a deeply weird party and you deserve all the pain you will encounter with it. (hint, you cannot buff or kick or shoot your way past Nessie)

There used to be a few character editors for Wiz8, they've either gone dark, or they are not compatible with this modified version.

Play the game it was meant to be played. Create a dozen parties of varied strengths and abilities, and plan for at least 11 of them to come to a horrible end. But oh that magic #12! It will make all worthwhile.

Yes, I know. Why would you think someone would come up with that if they hadn't already played a dozen parties? ^_^

I've tried the six-fighter party, the no-magic party, the no-spell bishop solo (didn't make it past the Swamps with that one). Just looking for a silly group to muck around with. Let me tell you, the number of panic buttons available with that group is very high indeed, and if you can't kill Nessie with just a valk and a samurai, then you've made poor choices all around =)

Honestly, Nessie isn't the fight you should throw in the face of a five-bishop party. Nessie is easy.