Difficulty stops boosting after NG7, but via a hacking device you can set it to NG8, and this causes enemies to once more have a large boost to HP, and a minor boost to all stats, with the exception of soul values.
The difference from NG7 to NG8 is that the first boss goes from doing 423 damage a hit, to doing 663.
-Clearly- I am intrigued. So I've reset my data, and I'm now going to play through a NG8 playthrough as my blaqk knight.
http://www.twitch.tv/flickerflare