I was just going to post a new thread on how awesome the 18v mod is, but I saw this one! I did the mod to my Schecter 7 string, and good GOD the tone is better!
What I was told is that the extra voltage reduces clipping, which is what my 2 lowest strings did every time I would dig in a little. I heard that as you go lower, it gets worse, hence why the higher strings are somewhat unaffected. I also heard that because they're more clear, there can be more high end fizz, which may be what some people don't like about it. Personally, I just make some adjustments to my amp and have no problems, but when used in the same set as another guitar with passives, it can be a little annoying.
Anyway, apart from sound, the dynamics are what I'm after. My 7 string used to feel dead and slow, but with the 18v in there it's got a new lease on life. I'll use them in a gig and see if I like them as much, but for now, they're NICE!!!