I have been asking around about this myself. Xtreme mode on X6 will take some time to get used to. I hear the enemies do more damage to you, but I haven't tested this myself yet. I was thinking about recording a run through normal mode and then playing it back side by side with a run of Xtreme to see how many differences I could find that way.
The Old School Video Game King!! http://www.youtube.com/user/Redmage1987
Yeah, the bosses have added attacks or added support. For example, Ground Scaravich has purple rocks that can't be destroyed on Extreme mode. Also, I swear that it is impossible to kill Infinity Mijinion with only uncharged buster shots on Extreme, as he ALWAYS just used his recovery move when I drained his health to 75%.
Not changing this sig until MegaMan Legends 3 is released. Started on 10/23/2011.