Here is information on difference between comp and non-comp. The biggest difference is the steering rack and stiffer suspension. You'll never notice the power bump as it's at the top of the rev range. 2016+ comp package is 600 hp instead of 575. Base M6 are all 560 hp.
https://f10.m5post.com/forums/showthread.php?t=873975
Regarding CCBs, IMO, it comes down to current mileage and how long you're keeping the car. Most used M6s will be around 20k-50k in mileage. CCBs needs changing around 70k or so. The problem is the cost for a full change is $12k. If you're not going to pay for it, the next owner will significantly discount the purchase price because you're giving him a $12k liability. You need to be more price conscience if you're buying a CCB car that already has 50k to 60k on the clock as you'll be the next owner to change the CCBs in a year or two. And no, you cannot simply mount steel replacements on, completely different hardware such as servo unit, calipers and misc bolts and clips.