I took a Z8 out on a demo ride about 3 years ago and liked the additional power and the smoothness of the twin vs the one-lunger but the import taxes down here are nuts, CFM did a great job on the Z6 design and because of that they have hardly changed it, even the Z1000 pictures I have seen look similar, I like my Z6 enough that I decided to redo it instead of buying a new Z and adding doors, windshields etc to it, with the flip up windshield and the rear window that I can roll up it gives me the option to basically ride with the machine wide open and get plenty of air or close it up in a hurry when the rains come and with the new mirrors and the rear cage modification I got rid o the 2 things I didn't like about the Z6 and now I think my Z6 looks about as good as the Z8, I actually like the roof lines better on the Z6 because it is lower than the Z8, for me there just isn't much difference appearance wise to make me move into a new one and while the added power would be nice for my riding it really isn't necessary, I ride alone and set my own pace and at this point in my life I am past wanting, needing, lusting over more power like I did in the past, right now I would choose a set of Elka stage 3 shocks over the added power of the Z8 in a heartbeat. To give you an idea of costs down here I attached a few adds my dealer had online today.
I pulled my differential out a few months ago not a big job, you might get lucky and find a used one cheap. We are caught in a bind with these older machines if they are down with a major problem, you probably need to almost give it away or repair it to sell it and if you get the repair done you might as well keep it especially if you have completed a lot of mods and are happy with it, I have decided that I like mine enough and with the cost of new/used ones down here that if I blow my engine up I will probably ship a new engine down here or send mine up north to get it rebuilt, I have no faith in my dealer down here to do much of anything. Good luck with your decision.